One of the most useful tools for .NET development is Lutz Roeder's .NET Reflector, now in version 5.  It uses reflection to peek inside .NET assemblies and disassembles the IL back into C# or VB code for those times that you need an in-depth look into any .NET library.  Ever wonder what the code for System.String looked like? This release adds some really nice features:

• Formatted code comments- doubles as a documentation browser for XML code-commented members
• 'Expand members' in disassembler lets you see code for an entire class at once- previously you had to look at each method separately.
• New analyzer features shows where classes are instantiated and exposed.
• Search Google or MSDN for a member

In addition to helping you debug code, this is also a great learning tool.  What better way to understand framework development than to peek inside The Framework itself?