Napoleon's tomb at Les Invalides
L'Hôtel National des Invalides was built by Napoleon Bonaparte as a hospital and a retirement home for war veterans, and now houses a few museums, including the Musée de l'Armée (the military museum). It's also the final resting place of Napoleon himself.
Napoleon's tomb is essentially a gigantic sarcophagus; seriously, the thing is enormous, which is somewhat ironic considering Napoleon's reputation for being a very tiny dude (which turns out to not be true at all; Google it, nerds).
I'm not particularly a fan of Napoleon or anything, but both his tomb and the building it's housed in are really impressive. Check out that ceiling!
Related junk from Cool Stuff in Paris:
The Army Museum at Les Invalides.The Panthéon La Place de la Bastille Chaos at the Arc de Triomphe