I used utorrent before. It's quite lightweight for me, until my ISP decides to throttle our internet connection. I use Deluge now. Its unstable, prone to crash every now and then, but it gets the job done. Plus no throttle effect.
I played Ragnarok Online few years ago and forced myself to stop. Then Lineage 2 (private), also forced stop. Earlier this year my MMO craving comes back and I end up playing Perfect World (MY).