Instant messengers are blocked at most work places, school's and universities using sophisticated firewalls and internet censorship applications, however such restrictions can be easily bypassed using web-based messenger services, allowing you to chat using your preferred IM service like Yahoo Messenger, GTalk, AIM or MSN without installing anything on your computer.
There exist several such online messenger services, here is a short list of best ones:
- Official Yahoo Web Messenger
- Official GTalk Chat Gadget
- Official MSN Web Messenger
- Official AIM Web Messenger
- Meebo - Multi Protocol Messenger
- ILoveIM - Multi Protocol Messenger
- eBuddy - - Multi Protocol Messenger
UPDATE: Try GPass for bypassing any type of internet restriction with ease, you can even watch streaming audio/video using the software - Gpass.