Before you get all starry-eyed about a PhD...
...read this. I like this essay because it is balanced; it isn't filled with horror stories about PhD's gone bad and neither is it all fluffy and rosy. The more time I spend in grad school, the more I begin to really understand what he is talking about; I suppose that is why it is one of the PhD guides out there. There are others like it, but this is one of my personal favourites: recommended reading, especially if you are seriously considering a PhD. (Don't miss the Feynman Problem Solving Algorithm, it's hilarious!)
Matt Might's advice for graduate students
Prof. Matthew Might at the University of Utah has written a number of articles intended for PhD students. Some of these are really insightful. One that got a lot of attention was The Illustrated guide to a PhD. Those worth reading for prospective and beginning PhD students talk about how PhD students might fail and 3 qualities of successful PhD students. This might be more useful to those who are a couple of years into their programme, while this list of recommended books and papers should be useful to everyone.
Mandatory link to Piled Higher and Deeper
This is a clinically tested and proven productivity-killer. You have been warned!
TeXClip is a nice tool for anyone who needs to use LaTeX in presentations (for equations), but doesn't want to use Beamer and doesn't use a Mac (read "doesn't use Keynote"). You type in a LaTeX expression and you get a vector image that you can save to disk and add to PowerPoint presentations, vector graphics drawings and anything else that tickles your fancy.