One of the greatest work that the Microsoft community is sharing on Codeplex is certainly the Application Architecture Guide project.
This is really a big project rich of useful informations. One of the greatest news recently addes are the Application Architecture checklists. I found it very helpful when you've to architect a new .NET application or you've to re-engineering an existing one. Having these checklists printed on paper helps a lot on making the right choices.
If you're interested, these are the links:
Application Architecture Checklists
Layer Checklists
Application Checklists