The module is simply called Free Order and should be painless to setup. Read through the page there and put it to use if you intend to use discounts or coupons on a site.
I haven't tested this in a setting where the products themselves cost $0, so I'd be interested in any feedback from those who have real world situations like that.
The only caveat is at the bottom of that page... it requires the latest code to work. I ended up fixing a minor issue with the order total JS which is post RC 1. I'll spend a few minutes tomorrow packing up RC 2 which will also include Lyle's fix to the product module's hook_access(). You can always just grab the latest Bazaar code to test it out in the meantime.