lsof - list open files

An open file may be a regular file, a directory,
a block special file, a character special file,
an executing text reference, a library, a stream
or a network file (Internet socket,
NFS file or UNIX domain socket.) A specific file
or all the files in a file system may be selected
by path.

Example, give you the list of processes using tcp port 80

lsof -i tcp:80