In my current small-ish setting, I use Trac for tickets and documentation. I keep most configuration files in a Subversion repository (an export of which serves as the depot for cfengine) -- and Trac makes it easy to link between wiki pages, tickets, and repository revisions.
I second the recommendation for Trac + Subversion for ticketing, documentation, + revision history.
I don't think this setup would scale to a large organization, but it works just fine in smaller environments.
For a large organization that needs a full-blown ticketing system, RT is applicable, but it's really cumbersome to set up if you just need a fairly small system.
Nagios, like cfengine, takes a while to get started, but does the job once it's up and running. Wolfgang Barth's book from No Starch Press can be handy to have around if you want an accessible introduction and reference.
And, for monitoring, I second the vote for Nagios -- very customizable.
M