A123 Li-ION RC battery.
I use Li-Poly batteries in my helis. Lithium Polymer.
They are dangerous. Dangerous enough that a company makes a product called LipoSack for charging and storing them (watch the vids). I store and charge mine in a metal box. Right now it is in the center of the cement floor in the basement. Yes. Yuck.
Helis use lipos because of their high discharge rate. When you buy an AA battery at the store it gives you 2 numbers. Volts and mAH. What is missing the discharge rate 'C'. That's what the fancy new AA batteries for digital camera offer. They just don't ever say the C value.
C really is a scalar to compute discharge rate. A 10C 2000mAH battery has a discharge rate of 10*2.0AH = 20amps. Depending on the vender that can mean continous or burst, but typically it is continuous. The battery I am about to buy for my new heli is 11.1volt 2100mAH 15C and 160grams. So 31.5 amps discharge. A LOT OF ENERGY that can be removed quickly and for not a lot of weight!
So people use lipos because they have rock'n discharge and are light. Unfortunately they are freak'n dangerous.
Most laptops use li-ion batteries. Lithium-ion. They are light and have a lot of capacity, but the discharge rate is not good enough for high-performance. (I dunno, I think I read 2C or something). They also have a couple nice features like they don't blow up and crazy enough.. are environmentaly-ish friendly.
Someone fixed the discharge rate of li-ion and they are competitive with lipos now. Awesome.
If I invested in companies. I would invest in this one. They just so happen to have a new line of RC products btw..
The prices are a bit high, but considering how new this stuff is, it's not bad at all. Also.. The current RC market motors are built around lipo voltages (7.4 and 11.1), but it won't take long for 6.6 to become king. Hopefully!