PBS' American Experience did a great piece on Jonestown last night (09/04/07).
For me Jonestown was the point in my then young life when the problem of evil reared its ugly head. It was also the first time that I discerned the sort of troubling things that groups of people can do given the requisite commitment. Indeed seeing the pictures of those bodies lined up as neatly as they were induced nightmares in me for days afterwards.
Note that I'm a great fan of the idea that our intellectual, etc. commitments are largely pre-political/pre-rational in nature. If I apply that idea to my reaction to Jonestown I could argue that the source my religious and political stances is in the events at Jonestown.