I've always greatly prefered MacOS X's Preview PDF viewing application to the bloated mess that is Adobe Reader, but it has a number of irritating features. The first is that I almost always find myself zooming in to a document to increase the font size for comfortable onscreen viewing. However, in Preview, you have to keep hitting the zoom button or shortcut until you get the size you want, then adjust the window size to fill the screen. It often takes quite a bit of manual tweaking before you get the window filling the screen. The preference item PDF > Auto-scale ought to do the trick, but it scales the document to fit a full page on screen, which makes the text much too small to read (for my eyes, anyway). Also, if you want to use Preview to present a LaTeX beamer presentation, you get an annoying 'onscreen display' for navigation, which is distracting and a bit unnecessary. Finally, if you use LaTeX it's inconvenient to use Preview to, well, preview the document, because it doesn't update the display when you edit and rebuild the PDF.
I've just found (via the TextMate mailing list) another application that fixes all those annoyances: PDFView. It's free and Open Source, it's fast and it's very nice to use. The auto-scale feature actually works, and it opens windows by default to fill the screen. There's a separate 'Fullscreen' view for reading without distractions (which shows just the document and scrollbar on a blank background), and 'Presentation' view, which fits the full page onscreen without scrollbars, so you can show beamer presentations properly, and it updates automatically when you rebuild a LaTeX PDF file, making it the perfect viewer to use together with TextMate when creating LaTeX files.
I think it says a lot for the quality of PDFView that after only a few minutes of using it, I changed the default application for PDF files from Preview to PDFView.