In order to measure current, there are several approaches to turn the current into a format that can be processed by analog or digital means.
Allegro produces single-package Hall effect current sensors with an integrated conductor from something as small as an SO8 for less than 50 A up to a unique package for currents up to 200 A. Even though the rated currents are peak values (and keep that in mind when you are trying to measure AC signals), 200 A must require quite a trace on a PCB. There are unipolar and bipolar versions available; in the case of the latter, the output is biased around the 2.5 V half rail. As with the LEM, the output is analog and has an accuracy of +/-5 percent.
When I suggested tackling this subject in the comments on a previous blog, EE Times member Brian@BDH suggested that I look at the Infineon TLI4970 High-Precision Current Sensor. This device has a very similar front end to the Allegro and LEM parts in that it has an integrated current conductor, and there are parts that will work to +/-50 A. It does not have a magnetic core, so it can claim an accuracy of 1 percent. In contrast to the others, though, this product has an onboard 16-bit analog-to-digital converter and an SPI interface. Unfortunately, neither the data sheet nor the device is readily available (Brian says it should be out January 2014), but it certainly holds a lot of promise.
Now, if you're measuring current, there is a good probability that you will also want to measure power. The Maxim MAX4211 combines a high-side current monitor (needing an external shunt) with a voltage monitor and an analog multiplier to create an analog power output.
This easily segues to single-phase and three-phase power/energy monitors for power meters. I'm not going to go there, partly because I have overstayed my welcome, but more importantly, I have not used any of them, so anything I discuss would be mere conjecture. Let me just say that -- just as with the aforementioned current monitors -- anybody who is anybody in analog ICs probably has at least one IC to address the power measurement market. Look for Analog Devices, Linear Technology, Maxim, Microchip, Cirrus, Silabs, ST, and International Rectifier, just to name a few.
Of course, I have omitted the basic electrics of magnetic interaction between a current and a magnet as used in analog meters, but I don't know of any electronic circuitry that can interface to this technique. Just to wind up, I wanted to point out an interesting lab instrument -- the I-prober 520 -- that will allow you to monitor the current in a trace without interrupting that trace. I have only tried it out once, but it does seem to work.
What have I missed? How do you measure current?