These guys blatantly copied
http://www.tslock.com (released November last year) - they even copied the tabbed design & all the features, favorites and all. Guess none of you guys care about that tho.
If you wanna save yourself the upfront cost & work you can try out
http://www.topshelfcash.com - we pay $20 per sale of TS Lock.
I'd be interested to know how the registration system works for this software, its tricky integrating software like this into payment providers like CCBill etc.. be interesting to see how this develops ;)