The Battle Grounds is a mod for Half Life or Half Life 2 (or anyone game runs with Source Engine).

"The Only Revolutionary War mod for HL1 & HL2"

http://www.bgmod.com/

Mission Briefing : Hunt down an invisible enemy.

The Infinitum Research Intercept Squad (I.R.I.S.) is on the hunt for a fugitive, the subject of a secret genetic experiment, who is known only as Subject 617. The Good News : The IRIS team is well trained, well armed and co-ordinated.

The Bad News : The genetic manipulation has given Subject 617 enhanced speed, mobility and incredible strength - he can pounce like a tiger, cling to any surface and is known to pin victims up for feeding. Worst of all, the experiments have rendered him nearly invisible to the human eye. This is why he is simply known as ... The Hidden

http://www.hidden-source.com/

BlackShot Online is a MMOFPS with exhilarating gameplay action, with a unique partnering system with partner's view unlike any other FPS games and is developed by Veritgo Games, published by Ntreev and highly recommended by Garena.

http://blackshot.garena.com/home

OpenArena is a Free Content project. Its goal is to create new, original media while providing a stand-alone multiplayer game in the process.

The current supported gametypes are as follows:
- Free for All
- Team Deathmatch
- Tournament
- Capture the Flag
- One Flag Capture
- Overload
- Harvester
- Elimination
- CTF Elimination
- Last Man Standing
- Double Domination
- Domination

Instant Gib and All Rockets weapons modes are supported.

Simulated players, or "bots" are also available for your personal pulverizing pleasure.

http://openarena.ws/smfnews.php

Infiniminer is an open source multi-player block-based sandbox building and digging game, in which the player is a miner searching for minerals by carving tunnels through procedurally generated voxel-based maps and building structures.

http://thesiteformerlyknownas.zachtronicsindustries.com/?p=713


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Minetest is an infinite-world block sandbox game and a game engine, inspired by InfiniMiner, Minecraft and the like. It has been in development and use since October 2010.

It is Free and Open Source Software, released under the LGPL, available for Windows and Linux.

http://minetest.net/

Download Ace of Spades now – it’s awesome, free and runs on any PC. It’s the best bits of Minecraft crossed with the fast-paced shooter gameplay of Team Fortress.

# Build anything – defensive positions to fortresses
# Destroy enemy structures, dropping opponents to their death
# Use your spade to dig through terrain – make trenches or tunnels
# Massive maps with lakes, rivers and tunnels
# Realistic rifle, shotgun and SMG including ironsights and recoil
# Dynamic lighting system based on ambient occlusion
# Up to 32 players battling simultaneously
# CTF or Territory Control game modes


System Requirements

# Your grandmother’s rig can run this game


http://www.ace-spades.com/

Diaspora... <3

World of Tanks is a team-based massively multiplayer online game dedicated to armored warfare in the mid-20th century. Throw yourself into epic tank battles shoulder to shoulder with other steel cowboys to dominate the world with tank supremacy!

http://worldoftanks.eu/

That game is getting a huge graphical and physics overhaul right?

Physics is coming in next patch, not sure about GFX though. It looks alright as it is ATM.

Runes of Magic (RoM) is a massively multiplayer online role-playing game (MMORPG) developed by the Taiwanese developer Runewaker Entertainment and adapted for the English and German-speaking market by German company Frogster Interactive. Frogster has also opened servers for France, Spain, Poland, Netherlands, and Australia as well as servers dedicated to the European Union. After going through an open beta phase, the game was launched on 19 March 2009 and Chapter II – The Elven Prophecy was launched on 15 September 2009. The next chapter, Chapter III – The Elder Kingdoms, started April 22, 2010, however all of Chapter III did not become available until August 11, 2010. Chapter IV - Lands of Despair, was released June 16, 2011. The latest Chapter, Chapter V - Fires of Shadowforge, was released on June 12, 2012. The game client is free to download, and no monthly subscription fee is required because the service is funded by real money transactions (RMT) in the Runes of Magic Item Shop.


http://www.runesofmagic.com/home.html

FYI, this game is VERY similar to WoW. Not that it is bad, but it is just very similar to it and borrows its art style and gameplay mechanics. I actually enjoyed it for awhile but stopped playing. It will be published on Facebook as a browser game later on as well :D

Exactly, from my opinion, this is a free to play clone of WoW. But I didn't know the facebook version, thanks for information!

Ready for Some Action?

Alliance of Valiant Arms is a high-octane MMOFPS that plunges you into the middle of a world at war. Side with the European Union (E.U.) or Neo-Russian Federation (NRF) and take up arms across vicious urban battlegrounds. Test your skills against your friends and see if you have what it takes to come out on top! Check out what AVA has in store:

Get Your Adrenaline Pumping

Powered by the Unreal3 engine, AVA offers immersive gameplay with HD quality graphics. Engross yourself in stunningly rendered battlefields and feel the heat of combat in lucid detail. Realistic actions and character movements bring the warfare to life as you fight to survive in the ruthless alternative history world of AVA.

Something for Everyone

Think you’re on top of your game? AVA provides a wide variety of game modes in which to prove it. Itchy trigger finger? Try your hand at classic gun-slinging modes like Annihilation or Escape, and mow your enemies down as they come your way. Master tactician? Try one of the more unique modes, like Escort, in which you must guard a tank and prevent its destruction as it trundles into enemy territory.

Go Solo or Combine Forces

Dominate the battlefield and eliminate the competition in Player vs. Player missions – or, if teamwork’s more your style, hook up with some friends and comb through intense, objective-based Co-Op missions. Endless challenges await you!

Customize Your Soldier

The right soldier for the right situation: Choose from three soldier classes with different traits and specialties to match your play-style – then gain the edge you need to prevail by unlocking unique skills and upgrades for your soldier.


http://ava.aeriagames.com/

INSURGENCY: Modern Infantry Combat (also known as INS) is a total conversion mod for Valve Software's Source Engine. Insurgency is a multiplayer, tactical first person shooter, and implements elements of realism, in an attempt to increase player immersion and promote teamwork.

http://store.steampowered.com/app/17700/