Sloth is a Mac application that displays a list of all open files and sockets in use by all running applications on your system. This makes it easy to see which apps are using which files and sockets.
Sloth is essentially a GUI built on top of the powerful
lsof command line tool, whose output is parsed and shown in a searchable, filterable table view with some convenient user-friendly functions such as revealing files in the Finder, killing a file's owning process, displaying file info, filtering by volumes, and so on. See screenshots below.
You can download the app here:
Sloth is free, open source software and has a home on GitHub.