I am a very happy user of FireFox by Mozilla and have recently discovered my newest favorite add-on:
ForecastFox.
It doesn't eat your PC resources like the weather.com thing that sits in your task bar. And if you have a browser window open all day (like I do as our support software is browser based), it's conveniently right at the bottom of the window all unobtrusive-like.
Our friend Gary is an IT guy and he was playing with Tim's new laptop this weekend (and broke it, hehe). Anyhoo, he was asking why anyone would want to use up memory on a second browser app like FireFox when the operating system already comes with a perfectly good one like Internet Explorer. (I'm all geek proud of my husband for knowing good reasons why). Tim said it was because most of the malicious security bugs prey on IE primarily, so using FireFox helps reduce that risk. I like it because it makes sense to me and I don't feel like it is secretly watching, tracking, and reporting on my every move. Plus, the fact that it is Open Source gives me minor warm fuzzies. And it is becoming more and more rare that I find a site that doesn't render or function just as well (if not better) than IE.
I do have to write the disclaimer that I have not done any extensive testing, but I haven't had any reason to, either.