Instead of collecting paper cards and fumbling through wallets at the cash register, customers are increasingly using their cellphones to track their visits and purchases, and receive rewards.

Some start-ups, like CardStar and CardBank, store existing loyalty cards on cellphones with scannable barcodes. And companies including Motorola and a start-up called mFoundry are providing retailers with the technology to build cellphone loyalty cards.

Loopt is one of several start-ups — including Foursquare, Shopkick and Gowalla — that are experimenting with ways to use cellphones to bridge the digital and physical worlds and turn the tasks of everyday life, like buying coffee and running errands, into a game.