2 of 21Image
Inspect element (built into Chrome)
As it turns out, the first extension I use quite a lot is the built-in function Inspect element, which you get to by right-clicking anywhere on a Web page. This replaces Firebug, which was an essential tool for developing and managing Web pages in Firefox.
Inspect element works just about as well, and it's always available.
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 absolutely critical and I couldn't live without it.