Cave Story 2

Initially, the player may only run and jump, but a variety of weapons and items may be acquired throughout the game. Most items are plot-oriented, having no impact upon normal gameplay, although a few, such as the Booster (a type of jet pack) and the Life Pot, are useful. Weapons include various beam weapons, a machine gun and a bubble gun, and can be obtained from treasure chests or traded for at various locations in the game. Though there are 9 different weapons, the trading system limits the player to a maximum of 5. Most weapons can be upgraded by collecting weapon energy dropped by enemies, and will lose weapon energy if the player is hit.
Source: http://en.wikipedia.org/wiki/Cave_Story