Well, I used to play a lot of MapleStory, but the grind takes forever once you hit Lv 40, most people then start paying for NX Cash when that happens, because of the 2x EXP cards.
But now, I casually (a word I'd never thought I'd say when I talk about gaming) play Osu! a lot, it's quite fun.
I usually hop onto by brother's computer to play some Team Fortress 2, which is probably the Best Online FPS ever.
But my all time favorite has to be Warcraft 3: The Frozen Throne, just due to the custom content it had (except for DotA), it made it a great game for online, but unfortunately, it's dying.