It has to be Team Fortress 2 becoming free. 331 hours in that game, no sign of it being boring yet. Well, that was until the game decided to trip over a rock and then smash its face on the sidewalk in terms of performance.
Terraria gets mention, at least. I've wasted plenty of time on that game.
Then, we have World of Tanks. (Also free) Which is host to its own slew of festering, maggot ridden problems, but has managed to just swing past the lowest two notches that define enjoyment on my 'fun' bar.
Lastly, I can sort of say Star Trek Online, like the fellow a page before me. Except, I played that game when it was subscription based only. It felt like there wasn't enough content then, and it feels that way now.