This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
Run netstat command on the command line. It will tell you which application is listening on which ports. I think the parameters to netstat change based on OS, although I might be wrong there. Look at the help for netstat