Interesting business models - how to make sales viral

I just found this site, and I thought it was an interesting idea.

http://www.mupromo.com/ (I am in no way affiliated with it, nor am I actually using it to buy anything. I don't even use a Mac.)

As far as I can tell, the idea is to sell a bundle of Mac software over a limited period of time. Some of the pieces in the bundle are "locked" and will only be given out to the buyers ("unlocked") if certain sales thresholds are reached before the end.

The incentive for users to convince their friends to also buy is obvious, while also being a lot more elegant than a simple commission.

I realize after typing this that it might seem slightly spam-like, but it's not. Now and then I find something that makes me go "hmm, that's neat", and this was one of them.