Meebo allows you to sign onto your IM client of choice (AIM, Yahoo, MSN, Google or unified through a Meebo account) from a web browser. It’s AJAX-enabled interface is pretty easy on the eye and equally easy to use. You can move windows around and configure it to how you like your IM client to appear.
It’s just IM, so it doesn’t feel like there’s a lot to say. I don’t value Instant Messaging particularly high, but this is a great option for those of you who do. It allows you the ability to IM on the go, or without having to install a desktop client, if that would be a problem.
Meebo has a few games and apps that you can play with people you’re talking to and it’s own community groups to meet new people. It offers quite a bit along with their convenient service. Check it out if it strikes your fancy.