Best Security / Vulnerability Testing Firms for Web Apps?

An anonymous reader writes "I'm in charge of a web application that must be extremely secure. Users will be submitting highly sensitive information to each other using the site. Security must be world-class. We believe we've built site in such a way that minimizes security risks and we've implemented numerous policies and procedures company-wide to increase security. We'd like a third-party to perform exhaustive and ongoing security tests: automated tests, application testing, and more, to check for things like cross-site scripting issues, server misconfigurations, form/hidden field manipulation, command injection, cookie poisoning, known platform vulnerabilities, etc. What companies would Slashdot readers recommend for these types of services?"