Arachni is a feature-full, modular, high-performance Ruby framework aimed towards helping penetration testers and administrators evaluate the security of modern web applications.
It is free, with its source code public and available for review.
It is multi-platform, supporting all major operating systems (MS Windows, Mac OS X and Linux) and distributed via portable packages which allow for instant deployment.
It is versatile enough to cover a great deal of use cases, ranging from a simple command line scanner utility, to a global high performance grid of scanners, to a Ruby library allowing for scripted audits, to a multi-user multi-scan web collaboration platform. In addition, its simple REST API makes integration a cinch.