Have a multi-author site, and for whatever reason one of the authors' avatar was not showing up. Installed this and solved the problem on the spot. Also nice if you want to use a different avatar for your site than you have globally, or if you simply don't want a wordpress.com/Gravatar account.
UPDATE: Plugin worked well until one of the 3.8 point releases, after which something broke. Had to disable in order for some of my users to show up on site. Am now using the Avatar Manager plugin.