Zeropaid has put together a great list of extensions to use for Azureus (one of the most popular BitTorrent clients). There are many BitTorrent clients that are available to be downloaded but these plug-ins are what distinguish Azureus from the rest. Some of the ones that I like on their list are the Progress Bar, Chat, and Safe Peer.
Progress Bar will actually add a progress bar to your data transfer page (duh!). The Chat plug-in will allow you to talk to other users about a specified file (useful for verifying integrity of the file). Lastly, the Safe Peer plug-in will watch out for IP addresses that are known to be the RIAA and other similar associations.