Watch the following video on NPR's "Science Friday" site (a lot of audio and video) about growing a rat heart. There's A LOT more information on stem cells. A few helpful links are below:
NOVA (Part A: Introduction "watch this segment", pros/cons, cloning and more)
Animations/cartoons on "what is a stem cell?" and "different types of stem cells"
General information
NPR's "Science Friday