Since there are a lot of suggestions here, I'd like to add my personal favorite: http://baretorrent.org/

While it does not have all of the features of more heavy torrent clients, it does have a plugin system - although not many plugins exist yet. It's cross-platform, open source, and pretty light. The author has good priorities and is conscious about issues such as privacy, and having good installers that aren't bundled with questionable content.