There are plenty of "free" Web vulnerability scanners including Wikto and Paros. Even the commercial vendors Acunetix and N-Stalker have free versions of their scanner tools. I have found that you definitely get what you pay for but these free tools can get you started down the right path. Just don't overestimate the value of scanners and underestimate the value of manual testing. The latter will uncover the *other* 40-50% of Web security flaws that the scanners can't find.
I would also like to make mention of the Web 2.0 security testing tutorial available on this site.
Here is a link: Web 2.0 application security troubleshooting, testing tutorial included in this tutorial are links to several free tools, explanations on how to use them as well as troubleshooting advice and videos.
This was first published in October 2009