Here’s one worth dowloading: Handle from SysInternals : it lists all the files currently opened and the processes they are opened by. While this facility has been present in Unix systems for ages, it took a while to get something similar it seems on Windows.
Have a look here.
Obviously, with the acquisition of SysInternals by Microsoft, this page is no longer available… or the utility by the looks of it — DAMN!
As it turns out, the page has been moved here nowadays:
http://technet.microsoft.com/en-us/sysinternals/bb896655.aspx