I'm looking for a good PC game?

I'm looking for a good PC game for me to waste my time on. Preferably on the simulation, or casual side. Vehicle customization is a feature that adds replay value to any game in my opinion, so I would love a game with that, like Spore, or Banjo Kazooie Nuts and Bolts. Online multiplayer adds replay value as well. I use Steam to buy most of my games so if I can get it there, that'd be nice. Thank you!
=EDIT=
Yes I have Minecraft, I love it, but I'm looking for something a bit different. And Warcraft sucks.

command and conquer general
red alert, alerd 3 w.e
the 2 games i just mentioned above are for online its free but the game it self isnt.
age of empire is good for single player.

these games are based on war and strategies.

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