Commercial version: Windows Live Messenger, AIM, Yahoo! Messenger
Open-source alternative: Pidgin
Pidgin combines the IM functionality of popular IM clients such as Windows Live Messenger and Yahoo Messenger into one lightweight application. This allows for seamless use of multiple clients and address books from the comfort of a single window.
The good: On computers with limited RAM, Pidgin does away with the need for multiple individual clients, which can greatly increase available memory. Multiple chat networks can be used simultaneously so there's no restriction on what networks you're connected to at any given time. It'll run on all the popular operating systems and a text-based alternative is available for anyone wanting to use the program within a console window.
The bad: It's not as attractive as the standard IM chat clients and it's not blindingly obvious which contacts are part of each network.
Conclusion: If you'd like all your contacts in one place, Pidgin is a superb option. Its functionality is flawless and features such as emoticons and file transfers are seamlessly integrated. It's not a particularly attractive program though, so if aesthetics are important to you, you might want to stick to separate chat clients.
Download it here: pidgin.im