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CSSViewer is another very helpful tool for Web development. Once you tap the toolbar button, it gives you a floating window with the CSS properties of anything you hover over. Love it!
How often used: I use this in spurts, when I'm doing Web development. Since that's not my full-time gig, it means I can go a month or more without using this.
Live without it factor: However, when I am doing Web development, this is very helpful and I couldn't live without it.