To be sure, not all user innovations end up in the marketplace right away. Just ask Felix Kramer, founder of the Palo Alto-based California Cars Initiative, a nonprofit whose mission is to bring plug-in hybrid vehicles to the market.

Kramer and other Calcars volunteers rigged a 2004 Toyota Prius with a battery pack so that it can be plugged into a 120-volt outlet and then operated on the resulting charge. "It's as if you added another small fuel tank to the car," Kramer says. That modification boosts the overall miles per gallon of gas of the Prius past 100.

But Kramer isn't interested in manufacturing plug-in hybrids. "Our whole goal is to get the car companies to do this; we promote awareness and enthusiasm," he notes.

However, given today's technology, the price tag for a plug-in battery system retrofilled on a Prius would hit about $10,000, says Dave Hermance, executive engineer at Toyota. "Plug-ins are an interesting concept, but they don't today offer a good value to ­customers."

That doesn't mean Toyota doesn't gain anything from Kramer's work. "It will still benefit by learning how many users pick up the idea," says von Hippel. If the number grows large enough, Toyota may decide it makes sense to pursue the concept.

TOYOTA'S RESPONSE TO Calcar's invention isn't unusual. Many companies are leery of working directly with users, if only because of the legal issues to consider.

Most companies go to great steps to avoid putting themselves in a position in which they could be accused of stealing someone's invention. That's why many prohibit their product-development employees from even looking at ideas submitted by individuals outside their companies.