If you're travelling this Christmas and using the wireless networks of hotels or other unfamiliar service providers, CanYouSeeMe.org is a handy way to tell what ports are reachable when you're behind a firewall.
CanYouSeeMe.org's open port check tool is a simple but useful web-based utility for figuring out if your ISP or firewall is blocking certain ports you need to get things done on your computer. It's a handy way to determine if connection issues you're having—like difficulty sending email or trouble accessing an instant messaging client—are a problem with your internet connection or the computer itself.
The paired down webapp is useful for running a quick check to see if a port you need is open. For a more robust look at your accessible ports, check out previously mentioned MyPorts to get detailed information on any open port, including its state, remote IP address, and app that's currently using it.
What are some of your favourite port troubleshooting tricks and tools? Let us know in the comments.