In a carrotmob, the "mob" decides which business to all frequent on a set date. They pick one by soliciting bids to see who will dedicate the largest percentage of their revenues that day to making their stores more eco-friendly.
This practice works by reversing the incentive structure companies usually face when making decisions about eco-friendliness. Instead of making money by cutting costs, they stand to make more money by spending more to be the greenest; instead of a race to the bottom, it's a race to the top.
Carrotmobs are almost certainly flawed--unless they somehow increase overall demand, they cut into business profitability with unknown long-term effects--but they do seem like a good way to make consumer's pocketbook-votes heard more clearly.
A comparison to Groupon is inevitable, but I think that carrotmobs avoid a lot of the problems that Groupon seems to be having.
(Thanks to Peter Jaworski)