Evince

The document viewer of the Gnome project can display many document formats beside PDF.

Skim

PDF reader and note-taker for OS X.

SumatraPDF

a free PDF, eBook, XPS, DjVu, CHM, Comic Book reader for Windows. Sumatra PDF is powerful, small, portable and starts up very fast.

Okular

The document viewer of the KDE project also displays postscript, doc, odt and other formats. It is well integrated with the KDE web browser Konqueror.

Xpdf

A lightweight, straightforward pdf viewer using a plain motif interface. The XpdfReader builds are non-free, we advise against them.

PDF.JS

PDF.JS is not a standalone application for viewing pdf, but a plugin available for various web browsers. It may suit you when you come across a lot of pdf files online or in an environment where you cannot or do not want to install a full desktop application.

qpdfview

A simple tabbed PDF viewer.

gv

A lightweight postscript and pdf viewer with xaw3d interface. Successor to ghostview.

sioyek

A PDF viewer designed for reading research papers and technical books.

MuPDF

Zathura