The WP Plugins website – refreshed daily it says – has a Top Downloads page where it shows plugins ordered by downloads per week. It also has a Trending page!
For an explanation of the purpose of the site and a brief overview of how it determines its results see the About page.
Thanks seem to go to Matt Gibbs ( @mgibbs189) for creating the site.
Note: The plugins.uproot.us site no longer exists.
For an alternative view see