Measure LiPo / Li-Ion SOC or the ever standing question of how much juice is remaining in my battery?

So I have a project in which an arduino runs on a two cell LiPo battery and I need a fast, straight forward way to determine remaining battery charge (with an accuracy of 10% so I only have a readout with 10 LEDs on it) without using any major extra components (so using an external battery charge meter IC is cheating :P). How should I go about this?

(using arduino mini, LiPo is two cells: 7,4V 150mAh)

Use a low-leakage op amp to divide the battery voltage by 8 (AV of 1/8). Connect the output to an analog input. Set your reference to the internal 1.1V reference (`INTERNAL`). Perform analog reads. Full battery will be about 975 ((8.4V/8)/1.1V⋅1024 ~= 977.45455). Empty will be about 700 ((6.0V/8)/1.1V⋅1024 ~= 698.18182). Note that the voltage scaling for Li-ion cells is non-linear; see the datasheet for the cells for the appropriate scaling (and if you want more precise full and empty values).