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.