Taskinfo2000 Ver 3.0.10.23
»This lets me see the number of windows/handles/threads a process has open.

Scott's Window Resizing Utility (SWRU)
»This lets me see the list of windows open on the system.

007x: Part of the Rix2K Extreme Power Tools freeware set. For the lastest versions, buy the whole set at RadSoft
»This also lets me see the list of open windows, but also buttons, forms, list boxes, etc. and displays them in a nicer way; in a tree view by parent->child relationship.

When I debug (want to actually watch a program execute) I use OllyDbg but I don't use it often, as debugging maxes out my CPU usage.