Pride of Li-ions
Top: official Canon NB-2LH, as supplied with EOS 400D. Replacement cost: £45 (or £37.99 at 7dayshop prices).
Bottom: 7dayshop compatible battery (with a slighty higher capacity at 750mAh). Cost £9.99. For TWO! Even if it explodes killing me and everyone else in a 100m radius, that's still a bargain.