Stan
The standalone judge

Where can I download it?

The latest version is Right Here

This is updated to fix a little bug in the display (thanks to Falk0n) but no change in functionality

This is a program which will sit on your pc and act like an externally hosted proxy judge / environmental script.

Upon loading it will attempt to determine your external IP.

Settings should be fairly self explanatory, but you shouldn't have to change them from the default.

Hit "start server" and it will run a few tests - telling you if there were any problems.

If you hit "Start" and the port is already in use, it'll tell you that. Another cause is a local firewall that restricts software from acting as a server.
If you get a message that the internal judge doesn't work whilst testing, it's a local firewall/software issue. You're restricting the access from the software.
If you get a message that the internal judge works, but you can't access it externally, then it's your router that needs configuring. Make sure to forward (through the router) the port you've chosen to run the server on.