I use Xolox. I have used Gnotella, Gnucleus and Bearshare and I like Xolox a lot, because it keeps continuously searching and updating a group of hosts having a file, and keeps downloading different segments from all of them simultaneously.

It is also very elegant and easy to use, and when things don't work, you don't get caught into an endless tweaking of parameters, because basically there aren't any.

The bad point is that on my Windows 98 box it will never run reliably for more than a few hours without a restart. It works much better in Windows 2000. Resuming everything after a restart is graceful, though.