I have built several current meters using Allegro hall-effect sensors. They offer perfect isolation and have no voltage drop. Depending on the model, they have an output of 20-133 mv/Amp. When measuring DC current, I usually use a rail-to-rail op amp to shift the output to near zero volts (the quiescent output voltage of the sensor is VCC/2) and amplify the output a bit before feeding it into the A/D converter.

Fantastic devices, very accurate and stable. Much neater than using sense resistors.