Application Architecture Guide 2.0
This guide was just released as a free download on CodePlex. The guide talks in-depth about best patterns and practices for designing applications built on the .NET Framework. I think anyone who is building applications, web, windows, etc., should reference this whenever they are thinking of building any application. The pdf download is pretty hefty read of about 380 pages but I think it is simple, short and consise on what developers/architects need to know about developing enterprise solutions.
You can download it below.