Ask YC: "Hacker-testing" service?

I've been working on a web startup with one other co-founder, and I am the only programmer. I try my best to make things as secure as I can, but it is always better to have another set of eyes looking over things to make sure I am not overlooking something obvious. Is there any security-auditing or "hacker-testing" services you would suggest that we can use? We're willing to pay something for this, but we don't have a lot of extra cash to throw around right now.

If you can't think of any services like this that you would use, what would you do in my place? Any advice is greatly appreciated - thank you!