I'm looking at buying this simple current sensing board: http://www.sparkfun.com/products/8883. Here is its schematic: http://www.sparkfun.com/datasheets/Sensors/ACS712%20Low%20Current%20Sensor%20Board%20v12.pdf.
The current sense chip outputs a voltage between 0 and 5v, depending on the current. The op amp on the board can then be adjusted to detect "very small current" changes. The op amp can be adjusted to have a gain between 4.7 and 47, but what does this mean? What is the minimum amount of current change the chip (link below) can detect?
The current sense chip is this one: http://www.allegromicro.com/Products/Current-Sensor-ICs/Zero-To-Fifty-Amp-Integrated-Conductor-Sensor-ICs/ACS712.aspx.
I'm using the Arduino Uno's ADC, which has the same range as the output to this chip. So, win!