lsof | more
This will list all open files and be a long list. To restrict the list, say, to all files on mounted volumes, use grep:
lsof | grep "Volumes"
I wanted an alternative to the useful windows "handle" command-line utility to locate open file handles. The new Snow Leopard finder tells you what program has open files on a mounted volume, but earlier versions of Mac OS/X won't.
The application name is the first item in the list and the file is the last.