For photographers who need longer battery life with Speedlite flashes, the standard has been very expensive Quantum battery packs. Typically around $300+ dollars (seen here via B&H). There are a few cheap external solutions that use 6x AA batteries (seen here on eBay), but they are merely extended power sources and often don't help to improve recycle times.
Now a popular product making rounds in the last year as an alternative is the GoDox external flash battery packs. GoDox also have a variety of other lighting products including large studio strobes and continuous lights. I had a brief look at these products over at the PhotoPlus Expo in New York, and these battery packs look very good. The Godox external flash battery packs are available for Canon, Nikon, and popular Yongnuo flashes as well found via eBay (Click here).
GoDox External Battery Packs for Canon Nikon Yongnuo Flashes