If you've ever used Rails page caching, you've probably run into the situation where you wanted to cache a page that had essentially static content, but couldn't because it showed a flash message. I ran into this recently, and thought it would be really slick to move the flash rendering from the server to the browser, so that the page content could be cached but the flash could still be rendered from a cookie. I mentioned my idea to a friend over at Pivotal Labs and he said "Oh, we did the same thing just last week." What they did was very close to what I wanted, and they were ready to release it to the wild, so I sat down with pivot Brian Takita and spent a little while working with him to extract the feature, document it and package it as a Rails plugin.
The result: the Cacheable Flash plugin. Brian's post has a good description, but also checkout the README. For the moment this affects the flash for all actions in a controller, but Brian and I are looking into :except/:only options to allow finer-grained control if that is ever needed.
ruby script/plugin install svn://rubyforge.org/var/svn/pivotalrb/cacheable_flash/trunk