From throwaway ideas tacked onto games that aren't much to begin with, to genuine labors of love - gifts from the developers to you - the world of the bonus minigame is wide, varied and rarely more than a couple of minutes long. Here're a bunch of our favorites.
Mario vs. Luigi in New Super Mario Bros
An insanely intense two-player battle that turns the classic Mario platforming action into a feat of endurance. The aim is to be first to collect up to 10 stars, but if the players decide to start knocking them out of each other then it can go on until somebody submits due to thumb blisters.
Mercenaries in Resident Evil 4
In terms of providing a long-term challenge, this is almost as substantial as the main game itself - you get to score points while an endless stream of enemies hunts you down. The shooting galleries are a bit more "mini" to be honest, but since you have to play through specific scenes to get to them, we're going for Mercenaries as the RE example.
Sort or 'Splode in Mario 64 DS
Separate the pink and black Bob-ombs before they go off. Simple enough at first, but when the "evil clown" fairground music speeds up and the most urgently fizzing Bob-ombs get lost in the crowd, you'll find that manipulating a stylus can actually make you sweat.
Tower Tennis in WarioWare Smooth Moves
In WarioWare's giant collection of tiny microgames, the more substantial ones really stand out. Our favorite is Tennis Tower, which is a sort of cross between Breakout and a platform game. Bounce your way up an infinitely tall tower while avoiding bursting enemies and other obstacles.