Under the code.sixapart developer site, there's a repository of plugins that many people are likely not aware of. Some of the plugins are outdated or still in development, but there are also a few goodies that have just never been packaged up for release. To get at these either involves checking the code out with a Subversion client, browsing through the directories while downloading each file and recreating the structure on your own machine, or an only slightly less-tedious URL hack.
I've had to do this a few times recently and got sick of the effort involved, so in the interest of laziness(and boredom), mashed together a bookmarklet to automate the URL hack: Plugin Repo Download. To use, just activate it at the top-level directory of any given plugin, like so for HelloWorld.
It's not smart enough to work its way back up there if you're already browsing down into a plugin's structure. I gave it a quick try and decided I didn't think it especially important, but might come back to that at a later date.
As far as compatibility, the bookmarklet works in Firefox 2.x, which is all that really matters to me at the moment. There's nothing so involved that it shouldn't also work in any reasonably current browser, but I haven't tested and probably won't have much inclination to do anything about it if a problem does turn up.