You begin life as a simple rolling ball, but through completing worlds gain eyes, then legs, then horns... and having some good solid fun all the while. Adding body parts allows you to access new worlds in the main hub area, and each world has three levels to beat, and then a short boss battle. The controls can be a little fiddly and you'll spot the occasional bug here and there, but in general The Creature is really well made and enjoyable.
http://thecreatureproject.wordpress.com/download/