Battery pack for speedlite flash
I use Eneloop rechargeables. They not only keep their charge over a long period of time, but are very fast to recycle between shots and go a long time between recharging. I can literally shoot with less than a second between shots with my 430EX.
Eneloops are made by Sanyo but are hard to find (at least around where I live). I now also use them in my wireless keyboard as well as my digital storage device. They are not cheap but neither is their value or quality.