Arthur Richards has written a Drupal plugin for RETS! This allows you to create a website that has listing information taken directly from the MLS. You can find it with other Drupal plug-ins.
Here is a clip of what Arthur posted on the VieleRETS mailing list about the plug-in:
I could use a lot of help testing this, and if anyone knows
PHP/MySQL/has experience working with Drupal modules, please help make
this better and get it to a point of stability!
Let me know if you're interested at all in testing/coding.
There has been talk about RETS as a WordPress plug-in too!