If you’d like to generate your own “Best of Tumblr 2011” collage like my New Years post, you can do so here.
The Best of Tumblr tool grabs all your posts and sorts them by popularity, grouped by year and month.  It can take a few minutes to load everything if you’re a prolific blogger, but the result is a pretty, reverse-calendar of your tumblr.  The most popular post is featured first, with three runner-ups that you can reveal by hovering over them.
Please note: This tool does not show posts that you reblogged, only your own photos.
Updated 1/2/2012:
Added “Advanced Options” to let people see popular posts from 2012 or non-photo posts
Sped-up loading
Fixed the “Ghost Reblogs” bug. If you reblogged someone who later deleted their account, tumblr turns the post into a normal post, not a reblog…so it was messing up stats.
Fixed a bug where the tool gives up when tumblr gives a 500 error.