Homebrew developer Danzel of AFKIM fame has released CTorrentPSP, a lightweight torrent client for the PSP.
Developed using PSPLINK, CTorrentPSP is designed to run under 3.X kernel based custom firmware. The feature set is rather minimal, with the author noting that advanced torrent protocol features such as secure HTTP support and peer exchange are not supported.
For installation instructions and detailed list of known bugs, make sure to refer to the included README.PSP file.
Download CTorrentPSP
CTorrentPSP - Torrent client for PSP (obv!) [Danzel]