Cool uses of Attributes or Annotations (CLR or Java)?

Question

What are some cool applications for custom attributes in CLR/C# code that you've done or heard about? Also interesting new uses of the standard attributes is also ok!

Edit: Since Java's annotations seems to be the same as CLR's attrbutes, uses of Java annotations is also valid.

Answer 1

• postsharp, which uses attributes to inject code (AOP)?
• [TypeDescriptionProvider] which can be used to provide a custom runtime property model - either completely different properties, or perhaps faster ones

And some core ones that are often overlooked:

• [TypeForwardedTo] - used to move types between assemblies without re-building
• [PrincipalPermission] - used to automatically enforce security on members