It seems that some Microsoft frameworks are only known by name and reputation, not by content or usefulness. Especially with the rise of open source frameworks they sometimes get a bit unpopular.
Which frameworks by Microsoft are not well known and are well worth exploring in depth? What is their reputation and area of usefulness?
I haven't used it much, but it's a fascinating topic: the Coordination and Concurrency Runtime. It's part of Microsoft Robotics Developer Studio, which is a pain in terms of installation and licensing, but it's a really neat way of looking at concurrency. Parallel Extensions doesn't quite cover the same ground, as far as I can tell - I'm hoping to see the CCR available as a standalone free library at some point.
The particularly neat aspect from my point of view is representing an asynchronous workflow via continuations in an iterator block. Not as neat as F#, but still pretty nifty.
Scott Hanseleman has a recent blog post on ELMAH: Error Logging Modules and Handlers for ASP.NET
Phoenix is an SDK for building compilers, code analysis and optimization tools. When released, it will be used as a back-end for future Microsoft compilers.
The SDK (pre-release) is available the above link.
More information available:
Wikipedia article
Andy Ayers: Understanding the Phoenix Compiler Framework (channel9 video)
I think you should check out the stuff they have on Microsoft Research. Like one of the frameworks i find really awesome is the NaturalSpec which is useful for testing.