Who makes the best non-editor software for Web designers?

Designing Web pages requires more than just a Web editor. To do it well, you'll probably need a graphics software tool, maybe a video editor, link checker, HTML validator, or something else. What non-editor tools do you use every day? Nominate them as best non-editor tool for Web designers. Only the 5 most popular sites will make it to the voting stage. And don't forget to nominate for other categories like Most Unusual JavaScript Script, Best Mobile IM Client, Best Free Email Service, and Best Reseller Web Hosting.