Second Life
Second Life is a 3-D virtual world

Second Life is a 3-D virtual world entirely built and owned by its residents. Since opening to the public in 2003, it has grown explosively and today is inhabited by a total of 1,932,425 people from around the globe.
From the moment you enter the World you'll discover a vast digital continent, teeming with people, entertainment, experiences and opportunity. Once you've explored a bit, perhaps you'll find a perfect parcel of land to build your house or business.

You'll also be surrounded by the Creations of your fellow residents. Because residents retain the rights to their digital creations, they can buy, sell and trade with other residents.

The Marketplace currently supports millions of US dollars in monthly transactions. This commerce is handled with the in-world currency, the Linden dollar, which can be converted to US dollars at several thriving online currency exchanges
Win 98/Me/2000/XP

Acrony
Compete online against other players

Think you're good with words? Are you ready for some non-standard fun? Acrony lets you compete online against other players to make the funniest phrase to a random acronym. Once everyone in the game-room has written their phrase, you then read and vote on the other players' phrases to see whose phrase is the funniest. It takes real wit and guile to become the Acrony champ, not to mention a sharp and sometimes devious sense of humor.
Win 95/98/ME/NT/2000/XP/2003

Prime Suspects
The Queen's Hope Diamond has been stolen!

The Queen's Hope Diamond has been stolen! It's up to you to search for clues throughout 22 levels, find the offending culprit, and recover the stolen gem.
Expand your investigations through the game's 29 unique locations by finding items that will unlock subsequent levels.

Thousands of clues are cleverly hidden within the beautiful illustrations, creating a new game experience each time you step into this masterpiece of visual indulgences!
Win ME/NT/2000 /XP

WarRock
Multiplayer online tactical first person shooter

War Rock is a Free2Play™ massively multiplayer online, modern tactical first person shooter (FPS) where players have the option to purchase a wide variety of gameplay enhancing items, services, and products.

Some of the highlighted features include:

Easy to Learn
Very low entry threshold, immediate access to action-packed frag-fests, even for casual players

Free2Play™
Unique Free2Play access and player options to purchase gameplay enhancing items, services, and products.

Frequent Updates
Frequent add-ons and updates that include maps, weapons, vehicles, and game-play modes-driven by Player Request.

Efficient System
An extremely efficient game engine that allows the incredible graphics and high framerates even on lower-end systems.

Variety of Combat Experiences
Close Quarters Battles between SpecOps teams, Platoon-level firefights in urban areas, or even Combined Arms engagements with Fast Attack Boats, Main Battle Tanks, and Helicopter Gunships, War Rock has everything any FPS-gamer will ever need.
Win 2000/XP

Conquer Online 2
Mythical and ancient role-playing game

In the mythical and ancient Kung Fu circle, there was an evil guild that had risen to power and dominated the once peaceful world. Living under its terror, a group of courageous men and women gathered as one to improve upon their lot.
Indeed, to fight against this evil.
To their purpose, they set forth to return justice back to the people. Acquire experience in this ancient adventure with all of its glorious magic.

Enjoy, for yourself, the endless and the most beautiful scenery yet created in a MMOG (Massively Multi-user Online Game). Interact with others in a mythical environment in which you develop your own character. Version 2 may include unspecified updates, enhancements, or bug fixes.
Win 95/98/Me/2000/XP

Knight Online
Medieval fantasy role-playing game

Come and indulge yourself with the medieval fantasy setting massively multiplayer online role-playing game (MMORPG) with the world's most exciting, heart-pumping PvP features.

Free2Play
Free2Play: All players have absolutely FREE access to all the current features of the game as well as future updates and expansion packages with no time limit.

Pay2Play
For die-hard fans who wish to support Knight Online financially and are interested in enhanced game experience (e.g., higher hit rates, increased player abilities, access to special weapons, and many other benefits), we offer a variety of premium options.

Global Popularity
Knight Online is the critically acclaimed medieval fantasy adventure game with perpetual war servers operating with local language-support in China, Taiwan, Korea, Japan, Malaysia, and North America.
Since its first introduction in Korea three years ago, Knight Online has been enjoyed by millions of players from over 80 countries.
Win 98/Me/2000/XP

OpenArena

This little game (with excellent graphics I might add) is built on the FOSS Quake 3 engine.
There is a single player, as well as multiplayer mode you can play with.
The latest version is 0.6.0, and is really a lot of gameplay packed into a small download (for what it is).
If you are a gaming fan, especially of Quake, Unreal, and other fraggin games like that, and maybe a little old-skool, you'll like this game.
I mean it doesn't get much better for free, does it? Heck, I like a free flash game, and this is just a tiny bit more involved.
Try it out, it is actually quite addictive.
Almost as much as the downloading of the game itself.
Win 98/2000/XP
Mac
Linux

The Battle for Wesnoth

The Battle for Wesnoth is a free, turn-based strategy game with a fantasy theme.

Fight to regain the throne of Wesnoth, of which you are the legitimate heir, or use your dread power over the Undead to dominate the land of mortals, or lead your glorious Orcish tribe to victory against the humans who dared despoil your lands … Wesnoth has many different sagas waiting to be played out. You can create your own custom units, and write your own scenarios – or even full-blown campaigns. You can also challenge your friends – or strangers – and fight multi-player epic fantasy battles
Win 2000/XP/2003
Mac OSX (see web site) 55 MB
Linux (see web site) 39 MB

Nexuiz

Nexuiz is a 3d FPS deathmatch game project, created online by a team of developers called Alientrap.

Nexuiz is based on the Darkplaces engine. The darkplaces engine is an advanced Quake1 engine developed mainly by Forest "LordHavoc" Hale, who has been working with the Quake1 engine for many years.

A few of Darkplaces main features are Quake3bsp support, realtime lighting and shadowing, new particle effects, advanced menu system, and Md3/Md2 model support. Because Nexuiz is based on the Quake1 engine, its source code along with its content is entirely GPL.

Nexuiz focuses on bringing the FPS back to the basics with quick, lethal battles and balanced weaponry. As of the current release there are 9 weapons and 24 official maps, as well as 15 player models, with an average of 2 skins each.

Nexuiz is primarily multiplayer (though it includes a full single-player campaign, which allows one to play through the various multiplayer game types and maps with bots), and allows for hosting and joining of games. It can also support new gametypes, or whole conversions quickly applied to it (much like Quake). Nexuiz supports most, if not all, Quake modifications (although with varying functionality). Currently there are a few custom mods and maps used.
Win 98/Me/2000/XP

Wizard Duel 2 

My entry for the Gamedev.net 4 elements 3 contest (came 2nd of 42 entries!). Featuring the elements Earth, Air, Fire and Water, and two Wizards who must battle to the death. This game can be played multiplayer over a LAN or singleplayer against a (reasonably crafty) AI.
Win 95/98/Me/NT/2000/XP

Xiah
Asian-themed martial arts game

Xiah, published by OnNet USA, is an Asian-themed martial arts massively multi-player on-line game.

This adventure is set in a fantasy world where four warriors battle the evil Blood Emperor in an epic struggle of good versus evil. Xiah allows players to choose between using a keyboard or a controller when playing the game. Players have the option to play with other players or alone and are able to upgrade weapons an unlimited number of times.

The quick reflex action feature gives players more realistic combat movement during battle situations by allowing continuous hitting action.
Win 98/Me/XP

Castle Wolfenstein, Enemy terrirtory

So you want to play soldier do ya? You want to learn to conquer your foes and be seen as a force to be reckoned with?
Well we here at PlanetWolfenstein pride ourselves on helping newbies just like you become the machines of destruction you need to be to survive, and in Wolfenstein: Enemy Territory, you're going to need us more than ever!
Now grab your gear and let's get to it trooper!
Win XP

2Moons
Save Haran from The Corruption

Based upon a successful MMO called Dekaron in Korea, 2Moons is set in an aggressive, dangerous world where players must battle their way through hardship to become the ultimate warrior. The game is directed by game development veteran David Perry
Win 98/ME/2000/XP