I have a Fenix L2D flashlight that I use for my headlight, and a Dinotte 140L that I use for my taillight. Both use AA batteries. I use rechargeable NiMH batteries, and the best ones I've found are Sanyo 2700s. I recently bought some Maha 2700s from Thomas Distributing because they are rated a little higher than the Sanyos and they were a little less expensive. I generally can run my lights 3-4 commutes (90 minutes each round trip) before having to recharge the batteries. I'm using a charger that I got on a special deal from Batteries USA when I bought the Sanyo 2700s.

I have also tried DuraCell 2650 and Rayovac low discharge batteries that are rated 2000. I use them as a backups because they don't hold a charge as well as the Sanyo and Maha batteries.

I've also got a Planet Bike Superflash as a second/backup tail light. It runs on AAA batteries, which also will charge on the charger I got from Batteries USA.