Meebo is a web-based instant messaging solution. Rather than requiring you to install a client on your computer, you handle all of your instant messaging affairs through the Meebo web site.
Registration is optional if you're using a single service like AIM, Yahoo! Messenger, Google Talk, or MSN Messenger; you can simply use your credentials for that service to log into it through Meebo. If you want to build a more fleshed out profile that logs into multiple services at once, you'll need to sign up for a free Meebo account. One bonus for people behind a corporate firewall, assuming that your firewall isn't blocking the actual Meebo site, is that as long as you can use secure web sites (those with the https:// designation), you can connect to all your favorite chat services.