VOICE CHAT APPLICATIONS
Using these can significantly speed up play of online game sessions, but all users will probably need to have a broadband internet connection. Most of these are available in both Windows and Mac versions.

Skype
Also supports video conferencing (which is not free)
http://www.skype.com/

Ventrilo
Great push-to-talk feature
http://www.ventrilo.com/

Gizmo
http://www.filehippo.com/download_gizmoproject/

Ineen
http://www.ineen.com/

TeamSpeak
(Note: the unofficial and unrelated "TeamSpeex" Mac client is still in beta)
http://www.goteamspeak.com/

Dolby Axon
Supports surround sound (but not in the free client, which is good for up to 5 users)
https://axon.dolby.com/

VIRTUAL TABLETOP (VT) PROGRAMS FOR RPGs

Below are links to every virtual tabletop program I could find, both free and commercial, so you can judge for yourself which tool best fits your needs.

Battlegrounds: RPG Edition
That's what the rest of this site is about. I'm just including BRPG to make the list complete.
http://www.battlegroundsgames.com/nutshell.html

Fantasy Grounds
Windows-only. Many people like the look and feel of this app, and the 3D dice rolling.
http://www.fantasygrounds.com/

KloogeWerks
Java-based. Steep learning curve, but powerful.
http://www.kloogeinc.com/

ViewingDale
Windows-only. Great zoom feature.
http://www.viewing.ltd.uk/cgi-bin/viewingdale.pl?category=main&sx=1280

D20Pro
Java-based, 2-week free trial license.
http://www.d20pro.com/d20/index.php

ScreenMonkey
Only the GM must pay. Players play free, using their web browser. The GM app is Windows-only. A Lite version is available for free.
http://www.nbos.com/products/screenmonkey/screenmonkey.htm

GRiP
No longer developed, but some people still use it and there seems to be a user community that supports it.
Note: Link updated for GRiP v4.
http://www.rpgrealms.com/grip

iTabletop Pandoren (v2)
Browser-based (requires MS Silverlight, Adobe Flash, and 1 GB RAM) Windows or Mac OS X. Features built-in voice chat, and streaming video/webcam support.
http://www.itabletop.com/

EpicTable
Windows-only.
http://www.epictable.com/

RPG Table
Mac OS X-only (v10.5.8 or greater). No online play in v1.0.
http://www.codechemistry.com/

3DVT
In beta.
No system requirements publicly available.
http://myvirtualtable.com/

3D Virtual Tabletop
Free while in beta, but will be subscription-based when released.
Runs in a browser. Android app also available.
http://www.3dvirtualtabletop.com/index.html

Roll20
Browser-based.
Free while in beta, but will eventually charge for storage space and artpack content.
http://roll20.net/

OpenRPG
Probably has the biggest user base of any VT app.
http://www.rpgobjects.com/index.php?c=orpg

OpenRPG+
This site hosts the development version of OpenRPG.
http://rpg.wikia.com/wiki/Installing_OpenRPG

MapTool
Java-based. Developers are actively enhancing this program regularly.
http://www.rptools.net/index.php?page=maptool

Gametable
Java-based.
http://gametable.mornproductions.com/Index

Interactive Dungeon
Windows-only
http://www.big12inch.com/id/

Dave's Flash Mapper
Possibly still in development.
http://www.csgmedia.com/files/david/fm/

RPG Manager
Windows-only. v2.0 is in development, and screenshots for it are now available.
http://www.rpgmanager.net/Home.htm

Playing Fields
Java-based.
http://gdiac.cis.cornell.edu/Projects/2006sp/PlayingFields/index.html

Dices
Windows-only.
http://sourceforge.net/projects/dices/files/dices/

RPG Tonight
Runs in your browser.
http://www.rpgtonight.com

Dungeon Helper
Windows-only.
http://dh.abomb.info/index.php

Avalomhilly
Windows-only.
http://www.avalomhilly.20m.com/

TTopRPG 2.0
Windows-only.
http://www.gamemodel.com/TTopRPG/index.htm

Rolistik
Windows & Mac OS X. In French.
http://rolistik.free.fr/

Virtual Tabletop
Windows-only. Note that the link below is a download link, not a website.
http://64digits.com/users/brighteyes/Virtual_Tabletop_v0.6_.zip

Virtual Game Table (VGT)
Windows-only.
http://www.siliconstorm.net/vgt/vgt.html

Irony Games' MapMagical Encounter Editor
Java-based, runs in a web browser.
http://pages.infinit.net/pdclarke/mmee/index.html

Map Maker (beta)
MS Silverlight app, runs in a web browser.
http://dnd.gudjonhrafn.com/mm/

Qex
Windows-only.
http://qex.skyfire.ca/

Masterplan
Windows-only. No online component
http://www.habitualindolence.net/masterplan/

Taulukko
Donation-ware.
http://www.taulukko.com.br/

Infrno
Unclear whether this is free for good, or free while-in-beta.
Windows & Mac OS; has video-conferencing feature.
http://infrno.net/base/welcome

RPGrounds
Unclear whether this will free when released, or just while-in-beta.
Will supposedly be a 3D VTT.
http://www.rpgrounds.com/

Live-Tabletop
Not even alpha yet.
Will support RPGs and boardgames
http://code.google.com/p/live-tabletop/

Fabletop (formerly known as Tabletop Quest)
Browser-based. Open beta.
Unclear whether this will free when released, or just while-in-beta.
http://fabletop.com/

MyRPG
Browser-based.
Unclear whether this will free when released, or just while-in-beta.
http://myrpg.ca/

tabledice
Runs in a web browser (Firefox or Chrome).
http://tabledice.com/

Discontinued, Abandoned, or Unsupported Programs:

WebRPG
http://www.webrpg.com/gamesystem/
Note: The link above seems to be dead, which to me indicates that this is a discontinued product, but I've found a mirror site here, in case you still want to check this out.

Tabletop Mapper
Windows-only.
http://www.tabletopmapper.hpg.ig.com.br/

Interactive Adventures
Wizard of the Coast pulled the plug on this when they revoked their D&D license, so these are no longer available as of November 30, 2006.
http://www.codemonkeypublishing.com/

GhostOrb
There was never an official announcement that this was cancelled, but the website homepage hadn't been updated since December 2005, and now the site is gone entirely.
http://www.ghostorb.com/

Vellum
An abandoned project that looked promising.
http://vellum.berlios.de/

Battle Aide
Free. Windows-only.
http://www.drakevision.com/index.html

DNDMapper
Theoretically in development, but no updates since December 2005.
http://www.eeconsulting.net/tmp/

Online RPG Battle Tool
Presumed to be an abandoned project. Probably Windows-only.
http://home.comcast.net/~r.soult/

V-FORT
Windows-only. The site is no longer available.
http://v-fort.org/

BattlePawn
The site is no longer available.
http://www.battlepawn.com/

RolePlayingMaster
Windows-only.
http://www.enworld.org/RolePlayingMaster/

YumiChat
Windows-only server, Windows and Mac clients.
http://www.rabidcomics.com/yumichat/index.html

TRIS
Text-based only.
Note: This site seems to no longer be available.
http://trisrpg.bronzeforge.com/index.htm

AjaxVTT
Linux-only. Not for the technically-challenged.
http://linux.softpedia.com/get/GAMES-ENTERTAINMENT/RPG/AjaxVTT-18260.shtml#

Tabletop Mapper
Windows-only.
Note: This site is no longer available.
http://www.tabletopmapper.hpg.ig.com.br

GlitterComm
Windows-only.
http://glittercomm.com

RPGWorkbench
Runs in a web browser.
http://rpgworkbench.blogspot.com/2007/08/content.html

Virtual Battlemat
Runs in a web browser.
Note: This site is no longer available.
http://virtualbattlemat.com/

BYOND Tabletop Gaming
Presumably still in development, but no updates since August 2005.
http://shadowdarke.byondhome.com/tt/intro.html

It's taken a lot of Google searches over many months to come up with this comprehensive list of virtual tabletop software. I compiled it so that anyone interested in VTs could use it to easily compare the various products, and it is now the most comprehensive VT list on the internet. I also hoped that by providing such a useful collection of links, that it would draw people to this site and they might have a look around while they're here.

However, more and more I am seeing my list reposted on various forums. I would rather people not do that, of course, but if you do, at least give credit where credit is due, and don't try to pass of the information as your own. Thanks.

Comparing and Choosing Virtual Tabletop Software:

Guide to Choosing a Virtual Tabletop
With so many VTs to choose from, it can be difficult to find the one that is right for you and your gaming group. This guide should help. To comment on the guide, please post here (forum registration not required).
(5-page PDF, 80 KB)
Link

VT Comparison Chart
This chart was originally hosted at the Four Ugly Monsters site. Now that FUM no longer exists, I have offered to host the VT comparison chart here. VT developers are encouraged to periodically email me updated information about their VT for inclusion in this chart.
Link

OpenRPG Wiki VT Comparison Chart
(Very outdated info, but includes VTs not shown on the chart above)
Link

Sites that cater to Virtual Tabletop Software users:

RPG Virtual Tabletop
A good source of information about many VTs, and content for VTs such as tokens, maps, and adventures.
http://www.rpgvirtualtabletop.com/

Four Ugly Monsters
Note: This site ceased to exist as of the end of September 2010. It was a great VTT-agnostic site, and will be sorely missed.
http://www.fouruglymonsters.com/

FUMcon (formerly known as iCon)
A virtual tabletop game convention, held once a year
Note: This site ceased to exist as of the end of September 2010
Visit the site

BOARDGAMING & WARGAMING SITES

BoardGameGeek
The ultimate source of boardgame and wargame information.
http://www.boardgamegeek.com/

Boardgaming.com
A less-intimidating (than BGG) boardgame site.
http://boardgaming.com/

Fortress Ameritrash
A haven for lovers of Ameritrash games.
http://fortressat.com/

ConSimWorld
A haven for wargamers.
http://www.consimworld.com/

Board Game Designers Forum
The place to go if you are interested in designing your own boardgame, card game, wargame, etc.
http://www.bgdf.com/