I use Pidgin, the default IM client on Xubuntu 8.04. But I also use the Department of Defense's AKOIM. And my laptop doesn't have a built-in webcam or microphone, so family uses Skype's IM. I really don't want three IM's open (one tying up a browser window and java!), so here's how I consolidated them into just Pidgin.
AKO Instant Messenger uses a standard xmpp protocol, so Pidgin can talk to it. All your contacts migrate like magic! If you have AKO/DKO access, get the details here. If you don't have AKO/DKO access, then safely ignore this paragraph.
Skype uses a proprietary protocol, so it's integration is limited. The skype4pidgin plugin (.deb package) shares Skype contacts and IM with Pidgin...but Skype must still be running alongside Pidgin to work...though many fewer windows must be open. After installation, BOTH Skype and Pidgin must be restarted. I had to edit the Skype options manually to turn off chat notifications - otherwise both apps whine when a new message arrives.