[Cool Tool]Unlocker : Unlock Files/Folders without going nuts
Process Explorer is good to find that out, but
Unlocker seems to have not only better usability for this task (Simply right click on the file/folder and select "Unlocker"), it actually has more abilities and ways to make a file/folder unlocked (such as unloading a dll, which ProcessExplorer does not do, I believe).
Jean Paul adds:
"It actually gets better though. If you try to delete a folder that has a lock on it, unlocker will automatically pop up and allow you to perform an unlock or a kill process. "
Cool tool Indeed!