Meebo - is out favourite, it is one of the original Ajax-based web instant messaging program which supports multiple IM services, including Yahoo! Messenger, MSN, Google Talk, AIM, ICQ and Jabber. Your network administrator probably has this one already blocked.
Yahoo! Messenger for the Web is a Flash based Yahoo! Messenger program from Yahoo!.
MSN Web Messenger is the web-based version of MSN Messenger.
eBuddy supports MSN, Yahoo, AIM, GTalk and MySpace network. eBuddy can also be accessed from your mobile phones, iPhone and PDAs. It supports chat conferences too.
KoolIM supports a wide range of IM networks including Yahoo, MSN, AIM, ICQ, Jabber, GTalk, Xfire and a few others. KoolIM does not require registration to use, but signing up for an account enables you to save your network IDs.
Mabber lets you chat with your AIM, ICQ, MSN Messenger, Yahoo! Messenger, Google buddies and also send messages. Registration is required to use this service. Mabber is also available in your mobile phones.
EasyMessenger supports MSN, ICQ, AIM, Yahoo! and Jabber networks. Registration is required.
imo.im is yet another simple web based messaging service that supports MSN, Yahoo, AIM, ICQ, Google, MySpace and Skype network. This is the only web based service supporting Skype.
Communication Tube supports four IM networks namely ICQ, GTalk, MSN Messenger and IRC.