There are a lot of instant messengers out there but most of them are sponsored by big companies like Microsoft (MSN/Live Messenger), AOL (AIM, ICQ) or Yahoo (Yahoo Messenger). There’s nothing wrong with that if you don’t mind some advertising… As I’m a fan of OpenSource software I’d like to mention Jabber (protocol is called XMPP) here. The server as well as the client software is open source and as far as I know the only limitation compared to other proprietary messenger protocols is the missing support of video chats. GMX and web.de released there own messengers based on XMPP, which means you can use any Jabber client to contact their chat network.
All you need is a Jabber account. A good point to start is the official Jabber homepage. There’s a list of servers where you may want to create an account. A Jabber ID looks like an e-mail address: nickname@jabber-server. If all is set you have to decide which Jabber client you’d like to use. There’s whole bunch of them and you can find some here. I’d recommend Psi. It’s available for all platforms (Linux, Windows, MacOS), easy to use and at least I don’t miss any features. If - like myself - you’re running Ubuntu with no KDE installed, which is needed by Psi you may find Gajim a good alternative. Or just stay with the desktop’s defaults: Kopete for KDE and Gaim for Gnome. When starting one of those messengers for the first time, they’ll ask for everything required to configure your account and you should end up witch something like this: