If you do .NET development, you owe it to yourself to use FxCop. This tool evaluates your assemblies (it analyzes the IL assembly, rather than the sourcecode itself), and flags possible deviations from best practices with varying severity levels. You can also easily create your own modules for FxCop, imposing team or organizational specific guidelines. All in all it's a very useful tool, and it's free. Even if you don't plan on following all of the framework guidelines, it's still interesting in that it points out nuances that you probably didn't know about.

This tool has been out for quite a long time, but I'm still amazed how many .NET developers have never heard of it, let alone used it, so I thought I'd drop some props for it.