Skim PDF Reader & Annotator

I read about Skim ages ago, but only recently got around to playing with it.

It's an OS X PDF reader that allows you to add notes. You can insert boxes, circles and lines. You can also mark sections of the text by highlighting, underlining or striking them out.

Skim provides panes for the PDF contents list, the PDF itself and the notes, as well as full-screen and presentation modes. All in all, an excellent PDF reader.