I just learned about $exception in the VS.NET watch window for .NET yesterday. This shows the current exception that has been thrown and is a big time-saver in not needing to find the little exclamation point icon and hover over it.
What other special variables are there in the watch window?
(Note: this question is about .NET, not C++.)
If you right click any variable in the Watch window, you can create an Object ID. This will give you a number, e.g. the first object ID will be 1#.
The Object ID represents the specific instance. The instance can thus be watched in the Watch window just like a regular variable, but you can keep watching the instance even when local reference go out of scope. When it eventually gets garbage collected you will lose access to it.
Using "@err" will display the value of GetLastError() and "@err,hr" displays the error message.
The $user pseudo variable is the only other documented one. In VS2010, the VB.NET debugger acquires some new ones.