Well, depends on where you're looking, try
http://www.portsdb.org.
A way I use to locally see what's going on is to use "netstat -an (-A inet)" to see what's opened, and then using "
lsof | grep TCP | grep (portnumber) to get the process Id (PID) of the app using that TCP socket, and correlate the PID with "ps ax -eo pid, args" | grep (PID) to show you what app is running using that port.
Ofcourse same goes for UDP or raw, just transmogrify(1) the greppings.
HTH