Just utterly free video games!
Black Mesa: http://www.blackmesasource.com/
Battlefield 1942: http://www.battlefield.com/battlefield-1942
Happy Wheels: http://www.totaljerkface.com/happy_wheels.php
Game footage used at starting of vid: APB Reloaded (free to play, however requires some paid upgrades to do nicely, so not within the countdown):
Non-game Images Used – Copyright (c) 2012 NukeNorway and its licensors. All rights reserved.
Music from – audionautix.com