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If you do any work at all with CSS, you'll love Pendule. If you're not a Web developer, Pendule will provide you only minimal value. Pendule is a developer tool that allows you to play with a lot of style sheet features, from turning them on and off, to removing certain attributes, to finding out information about what's on a page. I particularly like the color pointer tool, that lets me point at any spot on a Web page and find out its color specification.
How often used: When I'm in Web-developer mode, I use it a lot.
Live without it factor: I could live without it, but I'd prefer not too. It's very helpful, especially once you explore all the nooks and crannies.