1) Team Fortress 2. I have over 1200 hours invested into this game, it has so much depth. 9 different classes who all get different weapons and cosmetic items to use. Custom maps are possible but besides that, the official map database is big enough anyway. It has a whole trading aspect to the game with a full fledged economy and it has a competitive scene, with 6v6 teams and 9v9 teams. It's very fun to play in them.
2) Civilisation IV. This game lets you run your own Civilisation (maybe the title gave it away). I also call it the game where you see who can be the biggest annoyance to other civilisations, just randomly declaring war on them when they're in tight spots. Fun game, Steam only says I have seven hours logged into it but I played tons on the disc version.
3) Awesomenauts. Imagine if Metroid, Castlevania and DOTA had a freak mutant baby. This is the freak mutant baby, it's made by a Dutch company known as Ronimo and the objective of the game is like any other MOBA - take advantage of capitalism to crush your opponents and wreck their base by taking down their turrets. Unlike every MOBA though, this game is played differently - as a 2D platformer. This game is also available for PS3 and X360 but it suffers the same fate as TF2, the console version's now a completely different game because of update issues. The developers are still adding characters, maps and new gameplay features to the game.