>> X-Ray Goggles allow you to see the building blocks that make up websites on the internet.
>> Appmaker is a free tool for creating personal mobile apps, even if you don't know code!
>> Popcorn Maker helps you easily remix web video, audio and images into cool mashups that you can embed on other websites.
>> Thimble makes it ridiculously simple to create and share your own web pages.
In my opinion, Xenocode Browsers is the ultimat tool for checking your website in different browsers. Xenocode Browsers allow you to lauch IE6, IE7, IE8, Firefox 2, Firefox 3, Google Chrome and Opera directly from the web. The only weak point: The service isn't available for Macs and GNU/Linux powered PCs.
CodeBurner makes valuable reference material for HTML and CSS available right in your browser.
CodeBurner is a suite of tools for web developers, that provides reference material for HTML and CSS, integrated with a range of popular development environments.
1Install CodeBurner
2Type an HTML element or CSS property in the Search field, or click an item on the page
3View browser compatibility info, syntax, code samples and more