You may also want to check out <a href="http://compete.com" rel="nofollow">http://compete.com</a>.<p>If you can find a site that released their numbers, you can compare that site (your baseline) with other sites to get an idea of how popular they are, relative to your baseline.<p>While you won't be able to actually find out how many users each app has (unless they release their numbers), this can at least give you an idea of popularity. Hopefully.