VideoDownloader – Download videos from Youtube, Google, Metacafe, iFilm, Dailymotion, Pornotube… and other 60+ video sites! And all embedded objects on a webpage (movies, mp3s, flash, quicktime, etc).
StumbleUpon – StumbleUpon lets you “channelsurf” the best-reviewed [...]

I began doing some research regarding  plugins for WordPress, but I realized that  mashable.com did a great job of compiling a kind of best-off  for different uses:

50+ WordPress Plugins for Multimedia
30+ AJAX-Powered WordPress Plugins
30+ Plugins for Wordpress Comments
50+ Tools For The WordPress Admin
Now you just need the time to try them out…

Subster Rejuvenation v.021 is a WordPress plugin that turns your categories into subdomains. We are now running it on this site. I’m sure there might be a couple of bugs, so post comments if you find any.
Update 1-28-06
v0.21 is a major rewrite. Here’s a list of the new features:

Sub categories are now nested properly. So [...]