The way the Haxial one works is quite simple.
http://www.haxial.com/bribe/
and here is there current bribe list
http://www.haxial.com/bribe/brlist.html
I think it seems like a good idea, basically Haxial retains control over the main development, and they implement main stuff as their priority, but for features that users want, people chip in, whichever has the most money gets started on first, when that is complete the next one is started. They also have Bounty Hunters, basically people who are not Haxial but who will choose to write the code for a specific feature when the money is good enough.
This way more then one person can chip in, the list is very clear and easy to understand and all in all a lot of fun
EDIT: they collect the money in advance, no fancy software or pledging involved