Dunno how old this is, but I didn’t know about it anyway..
Super Mario War is a party game for one (which isn’t much of a party) to four players, which is somewhat reminiscent of the original Mario Bros. arcade game, except the only enemies are the other Marios, controlled by other players or the computer.
In Classic mode the object is to jump on the other Marios’ heads and keep them from jumping on yours, which is pretty fun, but if you get bored with that there are 16 other modes like Chicken, Tag, Domination, and Survival. A nice selection of levels, all based on Mario graphics, are bundled with the game, along with a level editor to build your own. Super Mario War is a free download for Windows or Xbox, though the latter requires that you have the ability to run homebrew games.