If you are referring to a site that you own, I recommend checking out the Samurai Web Testing Framework. This is a live CD that has the absolute best open source Web-testing tools. It is free, and all of the tools are compiled and ready to go.
Once you get the environment up and running, I recommend looking at w3af, a Web application attack and audit framework, and the Burp suite of tools, an integrated platform for testing Web apps. These tools check your applications for vulnerabilities like cross-site scripting, SQL injection and command injection.
Dig Deeper on Application Attacks (Buffer Overflows, Cross-Site Scripting)
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