OpenSourceCMS – You can test demos of CMSes, Blogs, Forums and other web applications on their site, the site resets their database and files for them all every 2 hours
CMS Matrix – Nice comparison and breakdown of CMSes, you can also pick features and search for CMSes that support those features
WikiMatrix – Very similar to CMS Matrix, except they specialize in Wikis
ForumMatrix – another breakdown of apps, this time for Forums
WeblogMatrix – breakdown of blog apps
PodCatcherMatrix – breakdown of apps that deal with podcasts
I had another link but can’t remember what it was, since I didn’t bookmark it (don’t you hate that?). Searching my FireFox history and Google didn’t bring it up either.