This looks like a great tool for building an HTML based Help system:  http://www.helpndoc.com

I discovered it when looking at the new help for BTDF (BizTalk Deployment Framework), and wanted to shared it with you, and file it away in case I need it in the future.

It’s free for personal use, but I don’t think its Open Source; the free version will apparently put banners in the various types of help documents created. The price is British Pounds for the standard and professional editions.