All are free or relatively inexpensive (the ones with a license fee are well worth it). Most are excellent learning tools as well as productivity tools.
LINQPad: I can’t say enough good things about this gem! It’s a scratch pad for .NET applications and so much more! The .Dump() method is fantastic!
.NET Reflector: Great way to look deep into the .NET Framework and more.
Balsamiq: I love this mock-up tool.
.NET Memory Profiler: Profile early, profile often.
ReSharper: I never go anywhere without this one!