In most unix systems you can use the lsof command which is normally available under /usr/local/bin/lsof. The only catch is you have to be root or have sudo access to run this command.
lsof -i TCP:
Once you find the process id, you can get more information about the process by using the ps command.
There is a developer works article that has good usage information about lsof.
If your system does not have the lsof command due to security restrictions, you may want to try the script found here.