I was curious what batteries to use for my Speedlight (SB700), and I was informed by many pro's to use Eneloop by Sanyo (Panasonic). As it turns out, these babies really do work well. They hold a charge through rigorous shooting, and have a quick rebound.
I was using disposable batteries in my Speedlight, and it was eating them like candy. Plus, if I left them in too long they tended to leak. But these Eneloop batteries are holding up quite well.
They're not too expensive either. I got them from (one of this site's sponsors) B & H. They have a starter kit for $34 that has all you need plus the charger.
http://www.bhphotovi...ck_Starter.html