Quit, start your own company, come to your former boss with an offer for a service he can't refuse. Possibly approach other companies who could benefit from this...
This is the option to go with if the thing they've built is a big enough deal to change business dynamics. No way of telling from the post, but something tells me OP isn't from that category. That sounds like a ballsy approach though.
Just make sure you can prove you developed it at home, not on ex-company time/hardware et cetera.