Fistful of Frags - Free 3D Western FPS Game

Fistful of Frags is a wild west themed multi-player first person shooter based on Source engine. The game has many game modes, advanced multiplayer bots for off-line practice and is free to play on PC/Mac/Linux. 

Install via Steam: steam://run/265630
Installer at ModDB: http://www.moddb.com/mods/fistful-of-frags/downloads/client-installer-39-full

Links:
http://www.fistful-of-frags.com/index.php

X-Men The Ravages of Apocalypse - 3D FPS Quake Engine Game

A total conversion of the Quake engine, based on the team of mutant superheroes X-Men, released in 1997.

Play the Game:
http://play.flaswf.tk/PTWBGC/Text/Series/QuakeEngineSeries/X-MenTheRavagesofApocalypse.html

The Dark Mod - Standalone Thief-Inspired Game Mod Based on Doom 3 Engine Source Code

The Dark Mod, PC Gamer's 2013 Mod of the Year award winner, inspired by Thief series by Looking Glass Studios, based on the Doom 3 engine source code, is a first person view stealth game in a Gothic Steampunk world.

The Dark Mod includes creative new gameplay features, dozens of unique AI, and a complete set of custom art assets, allowing players and mappers alike to enjoy missions in a gothic steampunk universe. When you play The Dark Mod, you feel like you’re playing a gothic stealth game... there's nothing left that's recognizable from Doom 3.

Download:
http://www.thedarkmod.com/downloads/

Links:
http://www.thedarkmod.com/main/
http://www.moddb.com/mods/the-dark-mod

Javascript PC Emulator - Linux in JavaScript

A very cool project by Fabrice Bellard, a Javascript PC emulator, which can even boot Linux. See Linux in JavaScript within your browser:
http://bellard.org/jslinux/

The Linux in the demo even contains a C compiler, an Emacs Clone and a small MS-DOS .COM launcher.