I can say unhesitatingly that every project on which I have worked has been a collaborative nightmare. The reason? Because Email has been the primary means of team collaboration.
I am pretty much sold on the Wiki approach to team collaboration. At this point, I have not personally implemented a Wiki, I am currently in the process of reading about the various options available. Needless to say, I am only interested in free / open source options. At the moment, I'm considering ScrewTurn Wiki. I am also anxious to see what Google does with their JotSpot acquisition.
In the meantime, to see why Wiki is good for collaboration, watch this video.
If you have any comments/suggestions on Wiki tools - do tell!