Xpdf

Xpdf is an open source PDF viewer for the X Window System and Motif.Xpdf runs on practically any Unix-like operating system. Xpdf can decode LZW and read encrypted PDFs. The official version obeys the DRM restrictions of PDF files, which may prevent copying, printing, or converting some PDF files.There are patches which make Xpdf ignore these DRM restrictions.
Xpdf includes several programs that don't require the X Window System, including programs which extract images from PDF files or convert PDF to PostScript or text. These programs run on DOS, Windows as well as Linux and Unix.

http://www.foolabs.com/xpdf/

BePDF

BePDF is a PDF viewer for the BeOS, Haiku & Zeta.
Besides viewing, it supports annotating and user-defined bookmarking for unencrypted PDFs. It's fully localized for 20 languages at the moment with additional languages being easily added via text files.

http://bepdf.sourceforge.net/

ePDFView

ePDFView is a free lightweight PDF document viewer using Poppler and GTK+ libraries.
The aim of ePDFView is to make a simple PDF document viewer, in the lines of Evince but without using the Gnome libraries.

http://trac.emma-soft.com/epdfview/

Evince

Evince is a document viewer for multiple document formats. It currently supports pdf, postscript, djvu, tiff and dvi. The goal of evince is to replace the multiple document viewers that exist on the GNOME Desktop with a single simple application.

http://projects.gnome.org/evince/

gv

gv allows to view and navigate through PostScript and PDF documents on an X display by providing a user interface for the ghostscript interpreter.

http://www.gnu.org/software/gv/

KPDF

KPDF is a free PDF reader based on Xpdf. It is integrated with the KDE platform, so it embeds very well in Konqueror as KPart. Feature highlights:

• Three different ways of searching: find dialog, thumbnail filter and type-ahead find.
• Capture images and text easily with kpdf by dragging a rectangle around what is desired to be captured.
• Choosing the default background/text colors, like a CSS style sheet.
• Ability to add bookmarks to pages.

http://kpdf.kde.org/

MuPDF

MuPDF is a lightweight PDF viewer and toolkit written in portable C.
The renderer in MuPDF is tailored for high quality anti-aliased graphics. It renders text with metrics and spacing accurate to within fractions of a pixel for the highest fidelity in reproducing the look of a printed page on screen.
MuPDF has a small footprint. A binary that includes the standard Roman fonts is only one megabyte. A build with full CJK support (including an Asian font) is approximately five megabytes.
MuPDF has support for all non-interactive PDF 1.7 features, and the toolkit provides a simple API for accessing the internal structures of the PDF document. Example code for navigating interactive links and bookmarks, encrypting PDF files, extracting fonts, images, and searchable text, and rendering pages to image files is provided.

http://mupdf.com/

Okular

Okular is a universal document viewer based on KPDF for KDE 4.

The last stable release is Okular 0.10, shipped in the kdegraphics module of KDE SC 4.4.

Okular combines the excellent functionalities of KPDF with the versatility of supporting different kind of documents, like PDF, Postscript, DjVu, CHM, and others.
The document format handlers page has a chart describing in more detail the supported formats and the features supported in each of them.

http://okular.kde.org/

Sumatra PDF

Sumatra PDF is a slim, free, open-source PDF viewer for Windows. Portable out of the box.
Sumatra has a minimalistic design. Simplicity has a higher priority than a lot of features.
It's small and starts up very fast.
It's designed for portable use: only one file so you can run it from external USB drive. Doesn't write to registry.

http://blog.kowalczyk.info/software/sumatrapdf/index.html

Vindaloo

A PDF Reader. It is currently under active development. Vindaloo runs under Mac OS X and GNUstep. The OSX version that comes as a dmg-file does not require installation of PopplerKit. For GNUstep, you need to download and compile PopplerKit first.

http://home.gna.org/gsimageapps/