The user-generated guides—like this one for the G1— document step-by-step teardowns of various gadgets to show you the exact hardware inside each. Apart from the appeal of gettting to dismantle and view your gadget's insides, the tool can also come in handy for hacks and repair jobs.
The site also offers guidelines for those interested in uploading their own teardowns (which you can do here), or you can browse existing teardown guides by most recent and popular category fields.
iFixit Gadget Teardowns