“FileMon monitors and displays file system activity on a system in real-time. Its advanced capabilities make it a powerful tool for exploring the way Windows works, seeing how applications use the files and DLLs, or tracking down problems in system or application file configurations. Filemon’s timestamping feature will show you precisely when every open, read, write or delete, happens, and its status column tells you the outcome.”
What I used this for was finding out which application had my file open when I was unable to delete, rename or remove it, which is a major pain in the ass sometimes. This wasn’t the best possible tool for that use, however. Unlocker is.