Windows only: Free program Process Explorer, by the good people at Sysinternals, is a Windows Task Manager replacement tool with all kinds of added functionality.
Ever wondered which program has a particular file or directory open? Now you can find out. Process Explorer shows you information about which handles and DLLs processes have opened or loaded.
The Process Explorer can be a great tool for diagnosing problems, especially when you’ve got tasks running and you can’t figure out what program keeps starting them. The right-click Google search of a process is an especially cool tool. If you prefer Process Explorer, you can set it to replace the Windows Task Manager so that the usual Ctrl+Alt+Delete will bring up Process Explorer instead by selecting Options -> Replace Task Manager.