Fstat

fstat is a command similar to the lsof command Linux users are familiar with. Simply put, it returns a list of all open files and what user context they've been opened in. Handy arguments:

fstat -f /usr/local/etc - only return locked files from the filesystem containing /usr/local/etc/ (/usr, on most systems)
fstat -p qmail-send - only return files locked by the qmail-send process
fstat -u jimbo - only return files locked by the user jimbo
fstat /usr/local/sbin/smbd - only return info on processes and users locking the file /usr/local/sbin/smbd.

see also: iostat, netstat