OWASP ZAP - Passive Scanning - Get Started
OWASP ZAP is an excellent (FREE) tool to test your website for common security issues. It has a large library of plugins and an what seems to be an active community. Although the tool has an active attack method, I prefer the passive attack method as you can use the site as you normally would. Although tutorials do exist on how to get started, I personally had difficulty finding them or knowing what I was even looking for when I first started. Especially related to passive security scanning. The interface isn't the most "intuitive", so I figured I would write these instructions for anybody encountering the same problems.