W3C Link Validator. Validate your site's links, useful one site rewrites etc. You can validate a whole site's links as long as there aren't too many pages.
Zenu Link Sleuth. Been around forever, zenu lets you check your sites locally or on the server. Not a bad little tool.
Netcraft :: What's that site running?. A convenient way to find out what OS and webservers run most sites. NetCraft.com has many other useful resources, like their longest uptime list. All bsd unix, except one Windows 2000 machine [this is because Linux rolls over it's uptime at about 300 days].
Robots.txt Validation. A very nice tool, will show you errors you may not have suspected.