Apart from floats, CSS Specificity is one of the most difficult concepts to grasp in Cascading Stylesheets. Smashing Magazine offers up a great article which attempts to make it clear (or at least, clearer).
Another attempt at the perfect reset.css file, this one from louisrofrano.com. As each browser has different default values, it is important to reset their default values to something that you can control.
YSlow is a great Firefox Add-on from Yahoo! that allows you to analyze various performance metrics on a website you’re currently visiting. Website mentalized walks through how he used information from YSlow to optimize his own website.