Winning the award for the least understandable UI metaphor: the ability to view the environment variables in the target when native debugging. This is yet another tiny undocumented feature of the native expression evaluator in VS. $env falls under the same extravagant disclaimer from the @clk post.
Place $env in the watch window and you get … the size in bytes of your environment block.
Far more useful is to type "$env=0", which evaluates to "0", and dumps all the Environment Variables to the output window. If you use the Immediate window "?$env=0" you get everything inline. The feature requires you to be native (or interop) debugging and to evaluate it from a native frame.
Despite the insane UI interaction, the end result is very useful information.