http://www.acunetix.com/vulnerability-scanner/
There is a free edition, that only tests your webpage against Cross Site Scripting (XSS) attacks, but be careful, it adds new entries into DB so backup your DB before running this scan.
But do you know any same type of open source altrnatives?
This seems to be a great program, but full licence costs a Lot.
Found this site:
http://sectools.org/vuln-scanners.html
but most of those top 10 programs are also commercial software.
https://addons.mozilla.org/en-US/firefox/addon/3899/
this is one Firefox add-on, but this doesn`t have an automated scan functionality.