Campfire
Facilitates real-time group chat and file sharing through any web browser.
Cryptocat
Browser based encrypted instant messaging that works on Chrome, Safari, Firefox, and Mac.
eBuddy
Provides simultaneous, integrated access to multiple instant messaging services through any web browser. With a version optimized for mobile access.
imo.im
Provides a web based interface to a variety of services, including Yahoo, AOL, Google Talk, MySpace, and Facebook.
LiveGO
Web-based platform for instant messengers providing convenient simultaneous access to multiple accounts through a single interface.
PHP121 Instant Messenger
Free web-based instant messenger written entirely in PHP. Operates in multiple OS environments, including Windows and Linux.
Yahoo! Messenger
Access to the Yahoo! IM service from any Java-enabled web browser.