I came across a couple of useful websites today which I thought I'd post here for others and for my own information.
First is Domain Tools, which provides the usual Who Is lookup and a whole lot more. Most useful I thought was the SEO text browser that shows how a website looks to a search engine and gives a few pointers to how your pages could be improved for SEO.
That site led me to About Us, that is an interesting attempt at generating a Wiki for websites. I imagine this could become an interesting site to browse and find new sites to visit. If nothing else, adding your site will add another inbound link to your site.
Another useful resource that I didn't find today but follows on from the SEO text browser above is the HTML Validator provided by the W3C. Running your site through this will show any problems with your HTML code. I've no idea if valid HTML will improve your search rankings but I'm fairly certain it won't hinder them.