A while ago Jay Fields described a number of ways to share Ruby code between projects and focussed on a useful technique which involves using RubyGems and unpacking them into your vendor directory. He also mentions the difficulties of versioning with Rails plugins.
We use a number of Rails plugins at Reevoo. Initially we used Subversion externals to include them in our projects, but for a while now we’ve been successfully using François Beausoleil’s Piston which is effectively an extension to Subversion. You end up with a copy of the plugin code in your own repository with the relevant revision number from the remote repository stored in your own Subversion metadata. Piston prevents you getting new versions of the plugin code every time you update (as you would with an svn:external) which can lead to unexpected changes, but makes it straightforward to update to a newer version of a plugin when you want.
You can find Piston here.