CometD is a scalable HTTP-based event routing bus that uses a Ajax Push technology pattern known as Comet. The term 'Comet' was coined by Alex Russell in his post Comet: Low Latency Data for the Browser.
CometD is a Dojo Foundation project to provide implementations of the Bayeux protocol in javascript, java, perl, python and other languages. Other organizations (eg. Sun, IBM and BEA) also have implementations of Bayeux that are not strictly part of the CometD project, but where possible they are linked to from this site.