Fuser utility can list IDs of all processes that have one or more files open. It's defined in IEEE Std 1003.1-2004.
This release will work only on FreeBSD 6.x/7.x. It may work, probably, on 5.x too, but I haven't tested it.
Some day it will be commited to main FreeBSD tree.
For now you can get it here, or install via FreeBSD ports collection (sysutils/fuser).
