Take a look a grc.com and follow the "Shields up" links. I'd provide a direct link to the appropriate page, but the owner has made that impossible "for our own protection".

If you can live with having to wade through Steve Gibson's "wisdom", this is one of the fastest and most reliable port scanners around, and the site has volumes of good information. Unfortunately, the good information is swamped in a greater amount of self promotion and opinion. Still very worth a visit though.

The idea of providing a web based port scanner that scanned a user provided IP number is problematic though. It would be so open to abuse. Every script kiddy around would start using it to indirectly scan his potential victims machines. The site hosting such a vulnerable tool would be rapidly shut down.