Markup PDFs without Acrobat

Marking up and manipulating PDFs is easy if you have Adobe Acrobat.  The Standard version costs a couple of hundred dollars, which I believe is worth it.  But, then, I use it a lot. If you have an Apple computer and want to explore some lower priced alternatives that are pretty good (but not as full-featured as Acrobat), check out this comprehensive article over at The Unofficial Apple Weblog which talks about two low cost products.   TUAW followed up that article with a short blurb about a tool called Skim, which is totally free.