For several years I have been wondering if there was a RDBMS that serves as a message queue as well1. And in the end of last year, noticing that MySQL 5.1 with support for pluggable storage engine was entering its RC stage, and that (from little googling) nobody was creating such storage engine, I dicided to write my own.
Q4M (Queue for MySQL) is a message queue that works as a pluggable storage engine of MySQL 5.1, designed to be robust, fast, flexible. The development started in late December of 2007, and although it is very primitive, operates quite swiftly.
Yes, it is primitive, and might have stability problems (I do not recommend using Q4M in a production environment), but if you are interested, please have a try.
1: At least Oracle seems to, but I never had a chance to use it.