HelpNDoc is a versatile application for building help files in CHM, HTML, DOC and PDF formats.
It includes a comprehensive WYSIWYG editor with support for hyperlinks, advanced paragraph formatting, embedded images in many formats (JPG, PNG, GIF, BMP, ICO, EMF, WMF), tables and more, while a spell checker traps errors as they're entered.
HelpNDoc makes it particularly easy to manage Help topics, attach keywords to them, organise them into categories and more. And it will look after low-level Help details like Topic ID and Context number generation, so you can get on with generating great content.
If you are also developing the project you're documenting, though, check out HelpNDoc's own documentation for useful advice on integrating the Help generation process with Visual Studio, opening an HTML Help file from a Java program, and more.
Please note, the Personal Edition of HelpNDoc cannot be used for commercial purposes, and adds a banner to the bottom of each page it generates. However, there's also a Standard version that removes the first restriction, and a Professional version that removes them both - follow the Buy link to find out more.