Flashes are incredibly convenient, but two issues have kept me from using them as much as I'd like; recycle time and battery life. Fortunately there's an inexpensive solution. It's the Flashgun Power Pack made by Pixel. What makes this power pack unique is that it uses standard AA batteries, the same as the flash itself. The pack holds eight batteries which along with the four already in my flash totals 12 batteries!
I ordered mine from FlashZebra.com and since it was just days before Christmas, I expected to see it well after the holiday. Nope, they shipped it Priority Mail right away which was a very pleasant surprise. It comes with the correct power cord (mine was for Canon), a case with a belt loop and a clever screw to attach it to the bottom of your camera using the tripod socket.
My tests showed that my Canon 550EX on full power recycled nearly three times as quickly as it does normally. The additional batteries extend the time between battery changes considerably as well. There's just one thing to be careful of; the reduced recycle time can overheat your flash if you misuse it. Flash Zebra recommends no more than 15 consecutive flashes before allowing the flash to cool, which shouldn't be a problem at all. Just be aware of it or you'll melt your flash. The same is true of any external power pack.
I'm looking forward to using my flashes more!
Pixel Flashgun Power Pack
Bought from Flash Zebra
Available for most Nikon, Canon and Sony flashes