The most useful API utility I've seen.
Dependency Walker is a free utility that scans any 32-bit or 64-bit Windows module (exe, dll, ocx, sys, etc.) and builds a hierarchical tree diagram of all dependent modules. For each module found, it lists all the functions that are exported by that module, and which of those functions are actually being called by other modules.
To submit a site or other reference work (offline media included), please drop me a line... Shawn K. Hall