Internet Relay Jabber (or IRJ) is a Jabber client for IRC. It's useful if you like IRC, but your friends use another IM service. Through Jabber, IRJ supports AIM, MSN, Yahoo!, ICQ, etc. It is written in Perl and uses Net::Jabber and Net::IRC.
Just to clear up any confusion, this isn't a transport or a gateway between Jabber and IRC (ie, a server component for many users). Basically what it does is connect to both Jabber and IRC as regular users. It then acts as an IRC bot to relay conversations in IRC to Jabber. It is meant for one user per IRJ process and pretty much requires a dedicated IRC channel. It also works best if you have access to a shell sever of some kind (so IRJ can always be running in the background) and use an IRC server with NickServ and ChanServ so you can restrict access to your IRJ channel.
That said, I personally find it very useful (though I may be biased because it's my creation) because I'm usually on IRC and don't like to have a bunch of other instant messaging programs open as well as my IRC client.