Here are some key resources for builing a repository. Hopefully I'll get a tutorial up soon.

Developer Applications:

http://forum.boxee.tv/showthread.php?t=16040 - Check out my repo with test apps in it for you to play with

http://www.fuzzthed.com/rss_link_builder.php - Flash/Silverlight Link Builder

http://www.fuzzthed.com/applications/js_interface_flash.php -  Javascript Boxee-App Creator (still under development)

Resources:

http://www.fuzzthed.com/pipestutorial.php - A Yahoo Pipes to Boxee Tutorial

http://www.fuzzthed.com/rsstutorial.php - My RSS app and repository Tutorial

http://developer.boxee.tv/application-repository/ - Boxee's Official Documention.

http://www.boxxxee.com/2009/05/creating-your-own-boxee-repository/ - Boxxxee.com Tutorial