open-source C PDF rendering library (and viewer) from Artifex Software; from web page, "MuPDF is ... small, fast, and yet complete. It supports PDF 1.7 with transparency, encryption, hyperlinks, annotations, searching and more. It also reads OpenXPS documents."
all OpenOffice.org applications allow PDF export; supports also PDF/A-1a; since many import formats are supported (e.g., doc, docx, rtf, xls, ppt) conversion is also possible
Legacy, a multi-platform viewer; based on Xpdf codebase
AmigaOS
Converters
Antiword a free MicrosoftWord reader for various operating systems; converts binary files from Word 2, 6, 7, 97, 2000, 2002 and 2003 to plain text or PostScript; available for AmigaOS 4, MorphOS, AROS x86
ClibPDF PDF Writer Linker-Lib for StormC (68k, PPC)
Viewers
gv a file viewer program included with the multi-platform Ghostscript package
Xpdf a multi-platform viewer for PDF files, Amiga version uses X11 engine Cygnix
Atari TOS
Porthos - commercial PDF editor, converter, viewer
Viewers
gv a file viewer program included with the multi-platform Ghostscript package
Unix / Linux
Annotation functionality
Okular supports a range of annotation types. Annotations are stored separately from the unmodified PDF file.
Xournal allows for complex and customizable annotations by freehand drawing, a shape recognition tool, multiple colours and text fonts. In addition, there is a rule tool for straight lines, highlighting and underlining. Annotations may be copied, pasted and easily moved. Annotations may also be in complex scripts such as Tibetan through SCIM input. Annotations are stored separately from the unmodified PDF file.
Converters
CUPS (Common Unix Printing System): supports exporting to PDF formats
Pdftk (the PDF Toolkit): can merge, split, en-/decrypt, watermark/stamp and manipulate PDF files
Preview, Mac OS X's default PDF viewer (in v10.5 it also can annotate, rearrange pages, and merge PDF files)
Skim, an open source (BSD licence) PDF reader and note-taker for Mac OS X
Microsoft Windows
Converters
ABBYY FineReader: Commercial PDF converter which converts PDF into Word (.doc), Excel (.xls), PowerPoint (ppt), and more
deskUNPDF: PDF converter to convert PDFs to Word (.doc, docx), Excel (.xls), (.csv), (.txt), more
GSview: File/Convert menu item converts any sequence of PDF pages to a sequence of images in many formats from bit to tiffpack with resolutions from 72 to 204x98 (open source software)
Global Graphics gDoc Fusion, proprietary, shareware; PDF views, edits, converts documents into PDF, XPS or Word; after 30 days a watermark is placed on documents in shareware version
Global Graphics gDoc Creator - proprietary/shareware enterprise-level creator to create, review, edit, share or archive PDF and XPS documents. After 30 days shareware version places a watermark on documents.
Bluebeam PDF Revu: Bluebeam proprietary desktop PDF authoring suite for office documents and technical drawings. Good markup summary and quantity surveying features.
Global Graphics gDoc Fusion: proprietary/shareware desktop product to create, review, edit, share or archive PDF and XPS documents. After 30 days a watermark is placed on documents in shareware version.
Infix PDF Editor: a proprietary PDF editor with word processor features
DigiSigner - freeware PDF viewer with additional function to digitally sign PDF documents.
Evince - a free (GPL), open source PDF reader. Part of the GNOME desktop environment. Windows port available since version 2.28.
Foxit Reader – freeware reader, supports FDF import/export, saving filled forms; other extended functionality available via purchasable plugins.
Global Graphics gDoc Fusion- proprietary/shareware to view PDF, XPS, Word, Excel, PowerPoint or image files, included in the evaluation version of the product. Shareware version places a watermark on documents after 30 day eval.
Nitro PDF Reader - free, proprietary PDF reader and creator. Supports PDF creation, saving filled forms (AcroForms), text typewriter, markup/collaboration, and stamp signature.
Nuance PDF Reader - freeware PDF reader, supports saving filled forms, partial support for XFA forms; enables the capability to convert PDF files to MS Word or Excel files through Nuance website.
PDF-XChange Viewer – freeware viewer, supports FDF/XFDF import/export, saving filled forms, extended markup and export to image capabilities.
Qiqqa – freeware PDF reader, indexer, tagger and annotator. Supports OCR and export of PDF text and images.
Sumatra PDF – a free (GPL), open source PDF reader. Supports automatic reloading of PDF files.
STDU Viewer - a freeware for non-commercial usage PDF reader. It also supports DjVu, Comic Book Archive (CBR or CBZ), XPS, TIFF, TXT and image file formats.