One of the things I really like about OS X is that it has a built-in PDF Viewer.  What's so special about this PDF viewer?  I think what I like the most is the fact that it is not bloated.  Adobe's Acrobat reader can take a while to launch, installing takes around 20 MB of downloads, and on older machines can really perform poorly.

I stumbled upon Sumatra PDF over the weekend, and I think it is a great replacemente for Adobe's Reader in Windows.   I think you cannot beat Sumatra, especially when it comes to the price (free!)