What software is available (preferably free) to edit PDF files on Mac OS X?

For simple tasks like fixing typos, OpenOffice or LibreOffice do the job

PDF creation is built into Mac OS X.

Open the app you want to create a PDF from and choose "File > Print" or "Cmd+P" and choose "PDF > Save as PDF..." from the dropdown in the bottom left of the panel.

You can then use the Preview app to rotate, crop, combine files, and more.

Have you had a look at Inkscape? It's a free, open source drawing tool. It allows importing of .PDF files and can save to .PDF as well.

Apple's built-in Preview application (free) alone sufficient to make small changes in PDF documents. When you double click on any PDF file, it opens, by default, in Preview. You can use Preview app to merge, split, mark up, annotate, and sign PDF files. Preview is the easiest solution to view, create and edit PDF documents on Mac.

With long documents or for complex editing tasks, third-party PDF editors like PDFPen or this one (cheap alternative to PDFPen) are the best bet.

I can't recommend any free solution since I don't use them, but want to second PDFPen as the best in class at any price. It looks like SamucoPDF is the least expensive app in the app store currently, but it has mixed reviews on the text editing.

If you really have more time than money, you might check out Stanza Desktop or Calibre both of which offer to accept PDF as input into file formats that are easier to edit with TextEdit or another editor you may have accessible. Once you've edited, you can print to pdf since the OS provides that to any app that can print.