Back in January I posted a list of which RubyForge projects own what RubyGem. That's all well and good, but it's a one-time dump of data and it's more or less hidden off in a corner.
So now to fix that, you can see the gem namespaces each project owns at the bottom of the "files" page. For example, here's the seattlerb project "files" page with their massive list of namespaces.
This list gets updated daily, so it should stay fresh.
Next task: the ability to transfer gem namespaces to another project. That's been a long time coming, but this gets us a step closer.