Developer Playground
Very cool tool...

"Developer Playground can only monitor interactions between a process and its loaded DLLs, and between DLLs within a process, but can't give any more detailed information about calls within the .EXE or within DLLs. Nevertheless, it is a powerful tool in the hands of a developer.

Typical usage would involve:

Spelunking — If you're ever curious what goes on behind the scenes, and why Windows is not behaving the way you'd expect it to, fire up Developer Playground and find out at least what the call-chain looks like.

Debugging — Do you have a problem that only manifests itself on some machines? No matter how much debug logging you add, you can't seem to track it down? Developer Playground shows you the program flow without your having to add specific debugging code or single-step through optimized assembly code.

Reverse-engineering — Are you trying to mimic functionality you've seen somewhere, but you can't figure out how they did it? Trace the target application and find out."