There are two models of Boeing's (Rocketdyne's) linear aerospike engine.  The XRS-2200 will propell the X-33 Technology Demonstrator currently being built by Lockheed Martin Skunk Works.  The upgraded RS-2200 is intended to power VentureStar, Lockheed Martin's next generation RLV.

The linear aerospike engine uses a common turbo-pump and a bank of liquid oxygen / hydrogen thrusters alligned along the top edge of a wedge shaped thrust structure.  The metal wedge acts as one half of the rocket nozzle and the ambient atmosphere acts as the other.  The key feature of the Aerospike engine is that, as the launch vehicle ascends during its trajectory, the decreasing air density allows the effective nozzle area ratio of the engine to increase.   As a result, an aerospike engine's performance is very high along the entire trajectory and is more efficient than it's "fixed nozzle" counterparts.