As described on the Cirt site:
[indent]Nikto is an Open Source (GPL) web server scanner which performs comprehensive tests against web servers for multiple items, including over 2600 potentially dangerous files/CGIs, versions on over 625 servers, and version specific problems on over 230 servers. Scan items and plugins are frequently updated and can be automatically updated (if desired).
It includes support for Unix (which would of course include OS X) and Windows-based machines and is the replacement for htmap from the same group.
This is an excellent starting point when performing a thorough review of servers under your administration for security, vulnerability and tightening up configurations.