I've been using Transmission for quite some time as my main bittorrent client, though the last version that supports Tiger is 1.54. However, via the MacRumors PowerPC Forum, I've found a user build of v2.31 for Tiger in this thread which supports magnet links, DHT, and UDP trackers.
I'm using it now and, though it's not a cocoa app and uses the Qt interface, I'm pleasantly surprised at the performance. It takes awhile to start up and doesn't support drag and drop, but other than that it performs the same as the old Transmission, using very little of cpu and memory resources. And it supports magnet links and DHT, two modern features that are becoming more and more essential.
The download is a universal binary, and though the first download in the thread is for 2.30b4, down thread a bit is 2.31 (