Windowblinds, IconPackager, and Winamp.
Those are the 3 that usually take the most work to create, so you typically get the greatest ammount of good new work, since there's a big incentive.
Arguments against other categories:
I think a widget category would be awesome, but odds are you'd end up with 90 cool clocks/mail checkers/weather checkers instead of anything really new or innovative.
Firefox is too much of a niche, as far as customization is concerned. Whereas pretty much everyone uses Windowblinds/IP and I'm fairly sure a lot of people use winamp over wmp, I don't think Firefox has even 75% of the userbase.