1. Google: Internet is just not the same without its search and map engines. 2. YouTube: No need for a cable TV subscription. 3. Wikipedia: Useful site to look things up in a hurry, especially articles relevant to academic subjects. 4. Dictionary (also Thesarus): Similar to Wikipedia, but when just looking up words. 5. Deviantart.
I've been playing since the engineer update (july 2010) in bursts. Except for source SDK which is free and I only use it to make maps in tf2 anyway, and me trying to get good at DOTA 2 (which I got via tf2 trading for 5 weapons and 2 halloween costume pieces, which while a good deal (roughly $0.15 for a $30 game) isn't too uncommon (usually goes for $1.40 as the absolute max via tf2, more often than not less than $0.40 (you'd be surprised how much money you can save buying through tf2)), tf2's pretty much the only thing I play anyway.