Google Talk

Google recently jumped into the Instant-Messaging fray with their Google-Talk service. It's based on Jabber, an open protocol that's been around for a while and works pretty well (I've been using it for at least a year) that has implementations around for every operating system you can think of (and in plenty of programming languages too so I'm sure you can use it no matter how obscure your requirements). It's also secure (encrypted messages), allows files to be sent to and fro, and planned to be extensible. Google already have their own ways of adding more functionality.

The easiest way for Windows users is to download their google-talk client for it, but anyone with a jabber client and a gmail email address can use it to chat and keep their online status available to friends.

Note: announced a few days ago, I've just been late publishing this.