LiFePO4 Batteries (lithium iron phosphate)
18650: $4.95 (3.2v, Protected, 1200 mAh, 39g)
You may remember A123 systems that became famous for their development of LiFePO4 batteries and their use in electric vehicles. When they went out of business in January of 2013, distributors scrambled to get stock of their cells. These cells were harvested from brand new equipment and never used, but they may show marks where the tabs had been attached and then removed. At 65mm long they will work in devices that only accept unprotected cell lengths. This is a very good choice for powering the single cell lights such as the bike lights that draw high current at 2800mAh. Stock of these is highly sought after and will not last long.
Use 2 of these in place of 1 18650 cell where you want brighter light in Spark Headlamps such as the SD6, ST6, and SX5. Two of them weigh just 30g (14g lighter than an 18650 cell). At 500 mAh you'll have to swap them out more often, but because they have a higher voltage output in series than an 18650 cell they will give the Spark headlamp higher voltage for more lumens output.