Foxit Reader

Foxit Reader is a free PDF viewer which is faster and lighter than Adobe Reader. It also (like Adobe Reader) easily integrates with Firefox (and probably other browsers) to allow easy viewing of PDFs while browsing.

Some of it's more advanced features such and annotation and drawing add evaluation marks unless you get the paid version (this is still better than some PDF viewers). As a general PDF viewer (which is what most people want) Foxit Reader is a good choice due to it's speed, which is what most people care about when trying to read a document.