Ummm... your own IRC server? Seems pretty realtime to me, and quite configurable. Of course, IRC is geared towards group chat, not user-user.

Perhaps include some more details about what your restrictions/requirements are - will the users be web-based? How many concurrent users will you expect online at any given time?

If you're looking for advice from others, it's best to provide as much info as you can.

Sorry for the lack of information, but the engine I'm looking for is web-based and able to do group chat or real time collaboration, and assume the engine can communicate with Ruby On Rails.

I looked into this previously and was interested most by XMPP. You can use open source servers like Openfire (Java, http://www.igniterealtime.org/projects/openfire/index.jsp) or eJabberd (Erlang, http://www.ejabberd.im/). It's more instant messaging than chatting, though (although it does support multi user chat)

Are you looking to write something, or use something like mibbit?

(In other words, would mibbit do what you needed?)

Can mibbit do realtime collaboration ? Actually the one I need is web based group chat and realtime collaboration, so other than chat the group can do realtime collaboration too.

Comet?

I search and found Ajax Push Engine, is it belong to Comet ? The exactly technology that I need is group chat or realtime collaboration, does this technology serve well ?