A golden oldie which I seem to need to lookup everytime I use it: the MetadataTypeAttribute.
When using entity framework to generate POCO classes for my applications I frequently run into the problem that I want to annotate the fields in order to utilize MVC Client-Side validations. The POCO classes are generated so they cannot be modified with loosing the changes upon a refresh of the generated code. The MetadataTypeAtribute offers a solution. Since all generated classes are partial classes we can add a partial class and adorn it with the MetadataTypeAttribute. The attribute points to a new class which mimics the EF class and provides data annotation to describe the rules and properties of each property.
Example:
