This may need to be moved elsewhere, and some of it has been covered before, but I picked this up from LifeHacker:
Use Technician's Codes to Diagnose Your Android Phone
Which links to:
Google Android Hidden Secret Codes - Tweaking with Vishal
Use Technician's Codes to Diagnose Your Android Phone
Which links to:
Google Android Hidden Secret Codes - Tweaking with Vishal

But I might have missed a few.