I've babysat mailservers before and don't recall it being all that difficult, but it's been a while (pre-mailman). I'd be willing to beat on this again; I would probably ask siderea what she uses for more current advice.

avacon uses something that looks vaguely like evite that's hosted on his own systems for invitations & RSVP tracking. I suspect it's the same DaVite that sprrwhwk mentioned.

On a related "I want this; is it already out there?" note, I've been halfheartedly looking for a wiki that will let me easily make a copy (read only is fine) to throw on a single machine and use locally only, preferably without having to set up a webserver on the single machine. (This is for Madrigal, where I both hate the google sites thing we're currently using, and would like to have this available read-only on site where there is no net.)