<p>Any free alternative to Robohelp?  Prefer open source</p>

<p>Need some sort of online help authoring tool for an open source project.</p>

Are you trying to make just CHM or other output formats too?
Take a look at DocBook. You can make (from one source file) pdf, html and chm - and some others, too. I've used it in the past but it's not very easy or convenient to use. If you only want to output chm (and need to use a free solution), see if you can get away with using the htmlhelp workshop (
http://www.microsoft.com/downloads/details.aspx?familyid=00535334-c8a6-452f-9aa0-d597d16580cc&displaylang=en
).