SIM Instant Messenger provides an plugins-based instant messenger with support for various protocols.
All protocols support richtext-messages, file transfer, typing notification, server-side contact list (with postponed synchronization - you can change contact list in offline mode and after log on all changes will be synchronized with server), new account registration, various searches and HTTP-polling.
You can use multiple accounts for each protocol: Oscar - ICQ and AIM support; Jabber; LiveJournal; MSN; Yahoo!