Knowledge tracking tools

[Steve Francis]

What do people use for knowledgebases?

I'm looking for a better (preferably open source) way to track change 
plans, event resolutions, etc.

e.g. an easy way to dig up what the changes that occured on a system 
were for, who did them, etc.
Obviously rancid et al shows us what changed when, but not the change 
plan that was responsbile or what problem it solved.

Possibly adopting a new ticketing system as part of this, so if people 
have built such system on top of RT, etc, that would be good to know.

Thanks