WHOIS service Domain based services allow you to see the following information: date of registration, name of registrar and much more.
BrowserCheck This service allows you to check how's your site look in different browsers (Internet explorer, opera, firefox).
Search engine spider simulator This service helps you find out for yourself how a search engine reacts to your pages. The search engine spider simulator provides you with a report on aspects of the page that are used to determine how highly in a search engine the page is ranked, and also gives some general usability feedback.
WhatTheFont You upload a picture with a graphic label, the service determines what font is using there.