GM Script: AllSizes+
UPDATE: 14th August, 2010
AllSizes has been updated to work with Flickr's new photo page menu. The information at the top of this thread is currently out-of date.
See this comment for details on upgrading to AllSizes v2:
AllSizes+ is a Greasemonkey script that gives you quick access to the different sizes available for a photo on Flickr. It produces better HTML code for posting photos on to blogs and Flickr discussion threads, lets you quickly view and download images, and does a few other things too.
AllSizes+ is intended for bloggers who (with permission) re-post Flickr photos onto blog sites and the Flickr discussion threads, as well as download images for offline projects.
See the discussion thread in the Flickr Hacks group for more details, and to ask questions about the script.