Openroom is pure-ingredients' chat application.
It is built on apache, mod_python , cheetah templates, and  uses ajax calls to provide a seamless chat experience. It supports private messaging and themes.
It does not use a database on the server, messages are stored in python marshal dumps, so you don't have to run a database server. Messages are automatically archived to xml files in a configurable message count.
If you would like to store messages in a database, it is easy to add a database storage interface to openroom.
Click here to try a demo of openroom.