Thottam Sriram, a PM on the CLR team at Microsoft, wrote a blog post a couple of weeks back about Managed Debugging Assistants (MDA).  MDAs are asserts (probes) in the CLR and in the base class libraries (BCL) that can be turned on or off; when enabled, they provide information on the CLR's current runtime state and on events that you as a developer could not otherwise access.  Some of these are extremely useful for debugging what might otherwise be some very nasty problems!  Thottam highlights an article we published in the May 2006 issue of MSDN Magazine devoted entirely to MDAs, Bug Bash: Let The CLR Find Bugs For You With Managed Debugging Assistants.  The article provides an in-depth look at a handful of the most useful MDAs, and it also provides detailed information regarding turning these probes on and off.  Check it out!