I recently discovered the great little gem. Pop in your Magento store URL and hit 'Scan' and it will check for known security issues that can be determined by hitting your website.
The great thing is that, since it was released, the authors have expanded it to add additional checks, and there's every reason to believe they'll continue to update it in the future as new vulnerabilities are discovered.
This is a great way to figure out if your Magento site has security holes, and if so, what patches you need to apply.
Check it out: MageReport.com