I play 'Robot Unicorn Attack' it kicks ass and now they have a heavy metal version mwhahaha.
Other than that I used to love Texas hold em until I lost all my moola.
Castle Age was another of my favorites.
I just started playing Total domination it is much like Command and Conquer but like most free games you have to siphon your money into it to buy stuff bleh.

I started playing farmville but got bored and now I despise all the 'ville games.