Bitlbee is an IM to IRC server, basically you use an IRC client to connect to Bitlbee which in turn connects to the IM networks. Just started using this, so I don't have much experience but it seems like an awesome program. If you want to try it there are public servers or you can setup your own server. It runs under Linux and probably other *nix's, is open source, and available in Debian's package repository.
The project website.
A list of public servers.
The project website.
A list of public servers.