Pokéwalk

From Pokemon - Wiki on Neoseeker

The PokéWalk is a device that you can obtain with the purchase of your Pokémon HeartGold and/or Pokémon SoulSilver games. The unit allows you to level up your Pokémon when you aren't playing your game like the Daycare Centre in the game. It can carry one of your Pokémon at a time, increasing your Pokémon's experience points (exp). You can also encounter wild Pokémon whilst walking just like a real Pokemon battle and have your Pokémon pick up items.It works by using a shake system which counts each time it shakes, or you step. These steps add up to gain your Pokémon more experience points, so you can be making your Pokémon stronger whether you're at home or at the park!