Play Coins are a core feature of the Nintendo 3DS. For every 100 steps registered by the system's internal pedometer, one Play Coin is generated, up to a max of 10 per day and 300 total. Various games, such as StreetPass Plaza, use Play Coins as currency to accelerate gameplay (such as buying Miis to play with instead of having to wait to StreetPass with one).
The trophy shop in Super Smash Bros. for Nintendo 3DS allows players to buy trophies with Play Coins at varying rates.