PSI/2 is a capable Jabber client aimed at experienced users. Its design goals are simplicity and stability.

The interface is very ICQ-like. Alerts appear in the contact list when events are received, and things like subscriptions requests appear as "system messages" (ICQ users know these well). Even "Chats" are queued in the contact list. Also, chats are "remembered" by default, so that you don't have to keep a bunch of windows open for each person. Just close the chat window. If you open it again it will all be there.

Psi is minimal but powerful. There are keybindings for just about everything, Unicode is supported throughout, and contacts are cached offline. Security is also a major consideration, and Psi provides it for both client-to-server (SSL) and client-to-client (GnuPG).