I've found a couple of links that contain lists of available add-ins for Media Center 2005 that I wanted to post here for folks to take a look at. I have been trying to get more familiar with the types of applications that folks are writing to extend Media Center functionality, but I haven't gotten to evaluate/experiment as much as I'd like to yet. I decided to get started by collecting links to different add-in repository sites and then expand from there. If you get a chance, please check out the permalink to my add-in repository article.
If you come across any links that you think I should add, please post a comment or contact me directly and I will evaluate them and add them if they look interesting.