Techstaff runs an Instant message server that is open to all with a Computer Science account. We make no guarantees on the uptime, but since we use it internally you can rest assured it will come back online in short order. It runs ejabberd and we have not had an issue with it yet (knocks on wood).
The details on how to sign in from various application will vary, so here is what you need to know.
Usually, the most you need to know to get started is the following:
Server Type: Jabber/XMPP Username: email@example.com Password: <cnetid_password>
In rare cases you may need to fill out the following:
Server: im.cs.uchicago.edu Port: 5222
Anything that will support jabber/xmpp will work just fine.