I've been using the free Foxit Reader for several months now, relying upon it in places where previously I'd rely on Adobe's Acrobat Reader (I believe they now call it simply "Adobe Reader").

While Adobe makes some fantastic software, Adobe Reader distracts from the actual documents by endlessly making attention demands, and you're eternally aware whenever it plays a part. It also tended to be very heavy on resources, with activities like scrolling through PDFs being a tremendously laborious process. Not to mention that Adobe Reader was responsible for quite a few browser hang-ups.

Foxit, by comparison, is extremely lightweight, and it manages to smoothly navigates through PDFs with ease. Great little product.