GAC Stuff

Richard Grimes Fusion Workshop (yes, that Richard Grimes)
Some sample managed GAC API wrappers from Junfeng Zhang
Documnetation of the (un-managed) GAC APIs

Other

Dependency Walker
Logging COM Calls by modifying the registry (I've had to modify the registry more in the last week than I've had to do in the last year, not because of COM logging but because of Biztalk and VSIP)