Nikto is an Open Source (GPL) web server scanner which performs comprehensive tests against web servers for multiple items, including over 6700 potentially dangerous files/programs, checks for outdated versions of over 1250 servers, and version specific problems on over 270 servers.
Some of the checks
Check for outdated server components
Apache and cgiwrap username enumeration
Mutation techniques to "fish" for content on web servers
Report "unusual" headers seen
Warning: This scan is produces a lot of traffic to the target.
While it should not take your server down, the traffic could be identified as an attack.
Please make sure you have permission to scan the target!