Find someone you trust to execute on it and share it with them.<p>I've done this and had this done to me; it's one of the two principle forms of social capital in Silicon Valley (the other being introductions).<p>Odds are someone else is already doing this or will be doing it within 3-4 months, if it's really a BIG IDEA.  Every "great idea" I've come up with in the last two years was launched by someone else inside 6 months (not always first).