For those of you that are working with integrating SharePoint with other technologies or just plumb curious as to the interfaces and technical specifications to take into account while developing solutions, best check out the 186 MB of SharePoint Products and Technologies Protocol Documentation that was recently updated and re-released. The original documentation was released back in July 2010.
Quick access here:
Microsoft SharePoint Products and Technologies Protocol Documentation – (1/10/11 v1.11)
The Tech Blog of Dan Usher 