Microsoft FXCop

Must download tool - similar to Java CheckStyle but more useful. Checks compiled assemblies for some coding style elements (such as use of CamelCase and method and field capitalisation), common OO errors (like default constructors in Abstract classes) and Framework errors and omissions (such as failing to declare Permissions and other attribute related errors). Very instructive - I learnt at least ten things the first time I used it.

As a side note, I'm not sure how people do it but this application has an icon that displays poorly at every single possible size and colour depth in Windows. Not bad going, but isn't the first...

Link