I’ve been looking at server security, testing things on my own web server, and recently found a company called Beyond Security who do free site testing. It looks like they go pretty in-depth with their automated checks and they give you this seal (which doesn’t always seem to load so it might look like a blank space):

Vulnerability Scanner

Their site is a bit flakey and the report you see when you click the seal as a website visitor doesn’t really reflect anything but as the site owner you get a detailed list of potential vulnerabilities.