OpenC1 is an open-source remake of Carmageddon. It uses XNA for the graphics and Nvidia PhysX for the physics. It implements almost all features of the original game, including vehicles, tracks, vehicle damage, pedestrians, opponents, checkpoints, UI, breakable, bendable track objects, animations. It does not currently implement powerups or network play.
As of version 1.4, OpenC1 can download the free Carmageddon demo content (artwork, models, etc) so you can experience OpenC1 without having to own the full original game.