Lutz Roeder's Reflector
is a class browser for .NET components (assemblies). It supports assembly and namespace views, type and member search, C# XML documentation viewer, reference search, IL disassembler, VB and C# decompiler, dependency trees, supertype/subtype hierarchies and resource viewers. Function prototypes are displayed in C# and VB syntax.
Also awesome list of tools from Scott
Via Jason Haley