A free UML modelling package.
Microsoft HTML Help 1.4 SDK
A very good tool (and free) for creating html help files, i.e. chm files.
This excellent application allows you to create pdf documents simply by printing to a virtual printer.
A free reader for pdf documents.
A free reader for ps documents.
Dimitri van Heesch
A tool for documenting source code by extracting interfaces and comments directly from the code.
Open source. Convert your documents into pdf files by printing to a virtual printer.
Free open source Office suite, that is compatible with Microsoft Office.
A free, open source, utility for generating documentation from source code.
A great tool for generating graphs from simple text representations.
Zeal is an offline API documentation browser available for Windows and Linux. Documentation for Zeal can easily be generated by for example Doxygen.
Online PDF Converter
This online service is easy to use to convert various file formats into pdf documents.