What is BitlBee?
BitlBee brings IM (instant messaging) to IRC clients. It's a great solution for people who have an IRC client running all the time and don't want to run an additional MSN/AIM/whatever client.
BitlBee currently supports the following IM networks/protocols: XMPP/Jabber (including Google Talk and Hipchat), MSN/Skype, Yahoo! Messenger, AIM and ICQ, the Twitter microblogging network (plus all other Twitter API compatible services like status.net)
There are also plugins for facebook and steam, and you can use libpurple to get even more protocols. See the wiki for a full list.
How to use it?
Just install the program and connect to the BitlBee server with your favourite IRC-client. You will be force-joined into the control channel where root (the bot, your assistant, the bee) will try to help you to get the program working.
As soon as you get your accounts working, you can add users to your contact list and talk to them, just like you normally do on IRC. Open a query, talk to the person in the channel, or even talk to them in groupchats that will look to you like just another IRC channel.
If you don't want to (or can't) install BitlBee on your own machine you can also use one of the public servers. If you use Windows, this is the preferred option, unless you use cygwin.