My weekend project this weekend was BundleWatcher, a tool that does just one thing: watches the gems in your Gemfile for updates. Once you upload your Gemfile.lock file, BundleWatcher will keep track of updates to the gems upon which your project depends, and you can use the atom feed for your bundle to know when updates have happened.
I built this to scratch my own itch. While rubygems.org provides a way to subscribe to gem updates and an RSS feed to track those updates, I wanted a way to track updates for each of my projects, project by project. Now instead of just knowing that the inherited_resources gem got updated, I can see which of my projects might need an update because that gem got updated.
BundleWatcher uses the (fantastic) API provided by rubygems.org to keep track of gem updates, so as long as you depend on gems that are listed there, you'll be good to go.