Documentation Software

Can anybody recommend software for documentation?  An online site would work but ideally I would like to keep the knowledge base local.  I'm not talking about sections of code.  I'm looking for process flows, file structures, what to do when X happens, etc.  The emphasis should be that it's structured and searchable.  Over the years I've mostly done this by putting it all in a single Word document.  That's hardly a database but it does allow for pictures and searching.  I've seen a wiki used to document a large system that was used by many people.  There was so much information it was unsearchable to get anything meaningful.  The structure made sense to the analysts but wasn't the best for developers.  I'm in a smaller setting now but would still like something that would have fit the previous environment.  Open source (free) is good.

Dave