PDF-Xchange Viewer is a free PDF viewing program that allows commenting on the PDF document, which the free Adobe Reader does not. The benefit of doing so, if you can get use to reading extensively from computer monitors, is enormous. Storing PDFs in your laptop frees you from carrying all those papers, and will save you years of back pain and medical bills in the future.
Furthermore, if you use Vista, the ability to search the text of the PDF papers all over your hard disk using Microsoft Windows Search makes looking up a paper easy. (Google desktop would do the same and runs on Vista and XP) Imagine trying to recall the author of a paper on bounding a utility function away from negative infinity in the midst of a discussion, searching for "bound utility" returns a list of papers containing those words and helps you find that person. The author I'm thinking of is Chad Jones, as you can see from the second visible result from the picture below. As you might notice from the picture, Windows search also looks through my OneNote notes.
The last feature I want to highlight is the availability of a portable version, which means the program works with no installation required. You simply unzip the program somewhere in your computer, and it will run. This is very useful when you have a campus account that doesn't allow for installation of programs. The notable missing feature versus Adobe Reader 9 is the lack of a text reading feature, but that can be be remedied with other programs. The software maker also have paid versions with more features - but sadly, I need that money for ramen.
More information on the program can be found on the product page here.
EDIT: Here is a much more detail review of the current PDF-XChange version (as of July 27, 2010)