Marble

a 2d game (with 3d graphics) where you roll around as a marble, collecting silver balls (or sheep) - collect them all to complete the level

your marble can move move, jump, and has two nice grappling hooks
old windows version supports multiplayer by plugging in multiple mice

Level editor/creator included

download from here

dist.zip is the new SDL version, but supports only 1 player/mouse
oldmarble is the other option