Related is hxxp://portscan.me/ - which does a nmap scan on the requester. Not a click-able just in case people don't read the purpose before clicking on it. Good for quickly finding out if your port forwarding worked (and it never works).
How do you plan to prevent abuse? E.g. this website allows someone to portscan another IP without exposing their own IP. Even with a per-IP throttle, someone with a botnet can use this website to effectively portscan any one host without exposing any of their botnet's IPs.<p>I've been using <a href="http://www.canyouseeme.org/" rel="nofollow">http://www.canyouseeme.org/</a> for this purpose. It does have the limitation that it only allows testing the ports of the connecting IP.
Technically, the page checks ports that are accessible on the public side of your local router. That can be very useful in checking one's configuration for errors and unintended vulnerabilities.