David Coallier has posted about his ideas for an Open Source, free PHP security scanner:

The idea of this project is quite simple, I was thinking that offering this service to anyone who requests it could be a very nice idea. Offering to run the scan is the only way I could ensure that people are not using this scanner against any website.

Basically, his idea is to offer this service free of charge, but still restrict it to avoid malicious use. It would involve validation via an md5 hash to ensure that the website has been authorized to run it. Check out the blog entry for more details on how the scan would be run.