I use a quantum Battery 1 if I need a LOT of flash shots in a short time frame, worn on a belt pack. It's old...but it works great, and the recycle speeds are pretty fast. Otherwise, I use Ansmann 2200 Mah rechargeablkes, which I recharge using two very sloooow chargers that take about 10 hours to recharge.
I realllllllly LOVE the newer Nikon speedlights liike the SB 800 (and newer) which offer the option of using the "fifth battery compartment" instead of the plain battery compartment door and just 4 AA's. Adding that fifth AA battery makes battery-powered speelight use just soooo much better!