The lsof program is a great tool to find open files. One case you might need this is to find a process that has an open file on a device you are trying to unmount.

List your own open files (includes sockets and pipes):

lsof

List all open files on the system (includes sockets and pipes), as root:

lsof

List open files for certain processes, as root:

lsof -c pattern
where pattern is the first few letters of the process name or a regular expression between forward slashes /regexp/.

List all open network sockets, as root:

lsof -i
Useful to make sure something unexpected isn't listening to the network.