Jabber is one of the several IM systems in regular use today. If you use Google Talk or the IM feature in Gmail, you've used a Jabber client.
My Jabber UID is psmay@halfgeek.org. If you're already using a regular Jabber client, you can simply add my UID to your list and see if I add you back. If you use Google Talk, you can either log into your Google Talk account using a regular Jabber client and add me there, or you can send me an e-mail from your Gmail account asking nicely if I will add you (Google Talk will automatically add me to your list if I add you to mine).
If you already use a multi-system IM client like Pidgin or Adium for AIM, MSNM, or Yahoo!, see if it also supports Jabber. If it does, consider getting a free Jabber UID from jabber.org and using it. It's usually as easy as adding a Jabber account to your client, entering your new user data (set "server" to jabber.org) and hitting "Register". Then, your new Jabber UID is screenname@jabber.org. Nothing to it!
If you are a Dreamhost customer, you can make Jabber accounts under your own domain name.