its called "Azureus" - which is the name of some frog or something.
basically, its the best BitTorrent program i've used. it's worked better for me than 'ABC,' BT++, and the single-file clients (official and shad0ws). i liked it so much, i even donated a couple of bucks to the author.
it's Java based, and runs in just about any OS i believe. and for Windows users, don't let the Java part scare you it doesn't look or feel like a Java app (like LimeWire), it "feels" like a native Windows app.
you can get Azureus here:
http://azureus.sourceforge.net/