UITest.com is a nice resource of online tools for web designers and site owners. The “Analysis” section has a collection of useful tools covering areas such as accessibility, linking, SEO and page validity.
Site Check
Of these, the tool I like the most is Site Check, which enables you to check a web site in a number of different tools without having to enter in the URL each time. Simple, but very convenient and useful.
Google vs. Yahoo! Comparison Tool
In the interesting but weird category is Yahoo! vs. Google, a tool which enables you to see how search results compare between the two search engines for a particular phrase.
If a site turns up in the first 100 results of both engines, it is represented by a blue dot which is linked by a blue line, enabling you to see the level of correlation.
I’m not really sure how I would use it, but it’s kind of cool.