Is there a user stats plugin available? I am looking for a plugin or simple method that can add some stats to the User Admin. We have a membership site running for a client and it would be good to add either a count to the sidebar User Groups filters (eg "Users (11)") or a dashboard widget of basic stats.

Has anyone added this already, or used something suitable?
Until the plugin store officially becomes a thing, Straight up Craft has got the most curated list of plugins available for Craft at the moment.

I'm not seeing anything like you're describing for users, but it is definitely something a plugin should be able to handle.