psi+

I always used psi as jabber-client for linux, but there were some feelings that it needs something more for comfortable usage. And couple weeks ago I've found that there are some russian guys who work on psi+ (based on psi) that already has lot's of customizable things. I really love it.

In Debian (and may be Ubuntu, but I haven't checked) it's available in repos, so you could install it:

apt-get install psi-plus