Now there's this new addition to Microsoft release schema: the Feature Packs. They seem to expose product features that didn't make it to the release. They get released to MSDN subscriptors for free, but seems to cost some bucks for others. They are also supported by Microsoft and are guaranteed to be included in the next product release.
Here is a good introduction to the filosophy of Feature Packs and discussion of the differences with Power Tools, Product Release and Service Packs.