Ask HN: How do I share an idea with a company but still receive credit?

It is a great little idea- that I would give away if it meant that it would be used. I do want to be in a position where I am able to take credit. What do I do?

Should I build a prototype? And then just start knocking on doors? Cold-emailing founders?

This is a complementary idea-- it would be something that companies like: SCVNGR, 4Square, Facebook and even Color.com will one day adopt to improve service for their users (and add value in their partnerships).