Nice work so far guys! I quite like Eclectix's, system404's, and Cy-AN's stuff at this point.
I'm definitely participating in this one. Basically, my concept is the free-swimming larva of a sessile creature, kind of like a sort of giant space barnacle that sends its offspring hurtling between planets and asteroids. It holds a symbiotic relationship with a number of creatures that it carries along for the trip. Intelligent species may clear out these symbiotic passengers, take their place, and chemically interfere with the host's systems to direct it to their chosen destination.