The new TH-320, from Leaf Buddi, is a cartridge battery mod.  The discreet design fits well in your hand.  They packed a 650mAh battery into the unit that uses Micro-USB charging for added convenience.  The variable voltage feature allows you to choose from 3 different voltages to give you the right amount of heat for your coil.  There is a 510 thread connection on the top of the unit allowing you to attach any 510 thread cartridge.  

Features
Fits Any 510 Thread Cartridge
Big Battery
Variable Voltage

Specifications
Battery Capacity:  650mAh
Battery Charge Type:  Micro-USB
Thread Type:  510 Thread
Voltage Levels:  3.5V, 3.8V, 4.2V
Dimensions:  2.95" X 0.87" X 0.59"

In The Box
1 Leaf Buddi TH-320 Cartridge Battery
1 Micro-USB Charging Cable