Linux only: It will only ever truly support Jabber/XMPP/Google Talk, but Synapse, a new alpha-level IM app, is a pretty—and pretty efficient—way to chat if you're all about open-source communication.
The developer of Synapse has a goal of spreading the love for XMPP, which is an open, extensible chat standard. That's great for the open-minded set, and users of Google Talk, which conforms to the standard, but is obviously a bit restricting, given the number of contacts one might have on "legacy" systems like AIM or MSN. There might be room in the future for server-side conversion of other protocols to XMPP, but for the time being, it's open-source or the road.
That said, even in a theoretically buggy alpha, Synapse looks great.
It works with Ubuntu 9.04's Growl-like notification systems, giving you chat updates in the upper-right corner of your screen without being intrusive. It can present chats in a number of styles, including some that ape iChat's bubble-y messages (at right), but with lots of colour customisation. The buddy list offers a pretty intuitive "grid" view that hides away usernames and statuses unless you mouse over—kind of a smart move for anyone who knows their friends' icons pretty well by now. And all the tools for in-chat sharing of pictures, files, code snippets, or even desktop views are available in a single, tab-friendly chat window.
Synapse also has a mind for webapp hook-ups. Don't like your icon, or got a non-existent one? Type a search into an integrated Flickr search box, and grab a pre-thumbed icon. Want to turn your Jabber "shouts" into Tweets, and re-purpose your Twitter posts as status messages? Throw in your user credentials, and you're on your way.
Synapse is an alpha-testing release for Linux systems only. Installation instructions are available for Ubuntu, openSUSE and