XNap is a Java plugin-based peer-to-peer (P2P) framework and client including an OpenNap plugin with multiple server support, as well as Gnutella, giFT (OpenFT), IRC, ICQ and an OverNet plugin. XNap provides resuming, chat, hotlist and a media library.
Simple. Useful. Light.
Lots of great options... too many to count. One of the better options that are out there right now.