As covered in last year's article on Preview's hidden powers, the OS X 10.5 version of Preview contains a lot of useful capabilities. One of the features discussed in that article was cropping images using the crop tool. While this is useful, I mainly use the crop tool for a slightly different task: chopping extra-wide margins out of PDFs.
Many PDFs—especially those that may have originally been formatted for print—seem to come with absolutely gargantuan margins. I've noticed a lot of wide margins on PDF versions of product manuals I've downloaded, for instance. On my large screen at home, this isn't much of an issue. But when I'm trying to read a PDF on a smaller laptop screen, the huge margins mean lots of wasted space. I can zoom in, of course, but that takes time and needs to be done differently for each document.
My preferred solution now is to simply remove the extra-wide margins using Preview. The process is quite simple. In Preview, open the PDF file whose margins you'd like to remove. Make sure Preview's Sidebar (View -> Sidebar) is visible, and that it's set to display thumbnails (using the pop-up menu at the bottom of the Sidebar).
With thumbnails visible in the Sidebar, click on the Select tool on the toolbar (or press Command-3, or choose Tools -> Select Tool). Drag out a region that encompasses just the region you'd like to keep, trimming away the excess margins. Leave that dragged-out area selected, then click in the Sidebar and press Command-A to select all the pages. (If your document has a mix of landscape and portrait pages, you can use Command-click and Shift-click to select just those pages you'd like to crop.)
With the proper set of pages selected in the Sidebar, and the dragged-out region visible over the selected page, select Tools -> Crop (or press Command-K). Preview will then crop all of the selected pages based on the region you laid out with the Select tool. After cropping, use File -> Save As to save your file with a new name. (I tend not to overwrite the original, just in case I want a wide-margin version.)
The whole process, assuming you don't have a complex mix of landscape and portrait pages in your document—takes only a few seconds, and I wind up with a more-easily-readable PDF on my laptop.