Finding Open Files
Sveinbjörn Þórðarson (Hacker News):
Sloth is a native Mac app that shows all open files and sockets in use by all running processes on your system. This makes it easy to inspect which apps are using which files and sockets.
It’s open-source.
See also: What’s Open and lsof.
Previously:
As said elsewhere:
The best application identify culprit to force quit it and fix the message "The disk… wasn't ejected because one or more programs may be using it” is "What's Keeping Me?"
https://www.macupdate.com/app/mac/27807/whats-keeping-me
Unfortunately, it has been discontinued. Hopefully someone could continue its development.