Skim - PDF reader and annotation software
Skim is an excellent free PDF reader and annotation software for Mac OS X. It allows you to do digitally what you are used to on paper: highlight, underline, add sticky notes. Beyond what you can do on paper, with Skim you can quickly search the PDF and browse your notes in a separate pane. Get it, try it, and stop printing out kilos of paper that then bog you down when you have to move.