fuser utility
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.
Sometime it will be commited to main FreeBSD tree. When I'll have time, I'll commit it to ports collection.