Elsword...
its like a 2d'ish action mmo brawler type game.
Think maplestory with less grind, faster paced, and more intricate (in the form of player made combos, some of which are really hard to time.)
One nice thing about it, is its very easy to automatch and jump into co-op "dungeons". Wait times are ussually only around a minute long.
One annoying thing about it is inventory management. They give you like 24 (i think?) slots for each inventory tab, and then you can unlock more with their ingame paid money currency. If you don't spend money to unlock more inventory slots, you'll constantly be unloading your inventory between dungeons and trying to figure out what you can get rid of that you won't need later.
I thought the game looked like junk, but then I played it about a month ago and was pleasantly surprised. It's refreshingly unique.
I've put about 100 hours into it, but it's charm is starting to wear off a bit. Still, for a free mmo, keeping me entertained for 100 hours+ is pretty good, imho.