(X)HTML Validator – Free service that checks Web documents in formats like HTML and XHTML for conformance to W3C Recommendations and other standards.
Google Trends – Great tool to see what search terms are hot and which was are not. Graphs show activity of a search term.
Google Webmaster Tools – Google webmaster tools is one place to spend some time. There you insert your site domains, and then have access to crucial data. Showing web crawl data, broken link data, and also gives you the ability to show where your sitemap is located.
Yahoo Site Explorer – Just like Google Webmaster tools… Not quite as nice but I do like how it shows the true amount of backlinks.
Find your google pagerank – Many say it is not important. I find it fun to check any way.
Technorati – Technorarti is only for blogs. It gives every blog a rank and shows how many incoming links there are to a post and when it was last updated. One day this blog will be numba 1. Well I can dream. Right now Engadget is #1.
Contact me if you want your website to be added to the list…