Meebo is an Ajax site that lets you chat online with your friends that use Yahoo Messenger, MSN Messenger, AIM, ICQ and Jabber (including Google Talk). You can log into all the IM networks simultaneously from your browser. If you don't want to create a Meebo account, you can get into one IM network at a time.
If you don't use VOIP or video chat features from the desktop clients, if you think they use too many resources and you don't like Gaim and Trillian, Meebo is the way to go. If you are at a public PC or at a computer that doesn't have Yahoo Messenger or the other IM applications or if the firewall blocks them, Meebo can help you get in touch with your friends.
I know what you're thinking: is it secure? Meebo encrypts passwords with 1024-bit RSA keys and decrypts them server-side using OpenSSL. If you want to encrypt all the traffic between you and Meebo servers you can choose https://www.meebo.com that uses SSL, but it's slower.
Related:
Gaim review
Yahoo Messenger 8 has plug-ins
Windows Live Messenger is out of beta
Tips for Google Talk