You may be thinking of fseventer, but it only shows changed files (not those just being read), and shows all changes not just those by a specific program.
Other possible not-quite answers: in Activity Monitor, you can "inspect" a process and (if you own it) see what files it has open. In Terminal, you can use the
lsof command to list files opened by all of your processes (if you're an admin, you can use
sudo lsof to see processes you don't own). Also check out the Terminal commands
rwsnoop. Finally, if you have Xcode (Apple's developer tools suite), check out Instruments.