Don't forget to run it as root, otherwise you might not see the programm that uses the port. -P will show the port numbers insted of words, and -n will not try to look up the host names and increases the output speed a lot.
sudo lsof -Pn | grep LISTEN | grep :80
Read this page for more alternatives and better explanation.
Update: I got a tip from Johannes for a solution that could be easy remembered by people who speak german.
sudo netstat -tulpen | grep LISTEN | grep :80
Tulpen is the german plural word of tulip. If you just use the singular tulpe the command will run very slow because it tries to lookup tha names for the ports and ip adresses.