When I still ran WinXP, I used uTorrent. It's closed source, and was acquired by BitTorrent, Inc, which is in bed with the MPAA -- I used the last version before the merger, just in case.
On Linux, I tried a few clients. The built-in client in Ubuntu is not suited for my purposes, and the others were quite clunky interface-wise. In the end, I settled for KTorrent, but that one crapped out on me a lot of times. Still, it was as close as one could get to uTorrent on Linux, so I made do.
Until a few weeks ago, when I discovered Deluge. It's an open source client, with packages available for many Linux distros and even Windows. It supports many features that were unique to uTorrent, has the same type of interface... it is, in short, perfect.
Well, there is a small thing left: when you close Deluge, it doesn't save your upload ratios -- so when you next start Deluge, all ratios are reset to 0%. Apparently this bug has been fixed in the source, so it is only a matter of time until an updated package is built with that fix.