WPAds is a WordPress plugin for showing and rotating ads in your blog. You only have to define the ad positions and the ads you want to rotate in each position, and WPAds will serve your ads randomly.
Google + Alexia Page Rankings display Google and Alexia Page Rankings in your dashboard and optionally on your WordPress website.
Registration Blacklist: Prevent registration and subsequent spam on your WordPress blog by preventing registration from blacklisted domains and email addresses. Although I am not sure that requiring registration for comments is such a hot idea, this plugin might be useful for those that have set that flag but are still facing spam problems or bogus registrations.