Cool Tool: Foxit PDF Reader 2.0
The free Foxit Reader standard edition is small and fast and lets you annotate PDF documents. No annoying "Welcome" screen while the reader loads in the background. They also have a mobile edition for Windows Mobile/CE devices.
But the best feature of them all; it is stable. I have lost track of how many times Adobe Acrobat Reader has crashed on my PC bringing Internet Explorer down with it. Foxit Reader, on the other hand, is rock solid. I have used the 1.3 edition for a long time and I just upgraded to 2.0.
The pro pack ($39) is needed to save modified PDF files or convert the whole document to a simple text file.