Lace is a free chat application for your website. It takes advantage of Ajax in modern browsers, and degrades gracefully in older browsers.
Lace doesn't care if you prefer Firefox or Lynx.
Update (Feb 25, 2009): Lace is now on GitHub for posterity: http://github.com/brettstimmerman/lace. The GitHub repo was created at version 0.1.5.1.
Update (Jun 11,2007): The Lace development blog and forum are now offline.