Minecraft is good if you're feeling creative
Team Fortress 2 is completely free on steam
I've heard really good things about Skryim from friends but it never really caught my fancy
Many games for xbox and ps can be played on PC too
lego batman 2
I could suggest you read a book instead, but I doubt you'd like that.
Rage will work on even the wimpiest PC, it's cheap, and has unique graphics.
Without knowing what kind of PC you are playing on it's impossible to recommend many modern games.