For various ranges like 5A, 8A, 20A, 30A, 50A, 100A, 200A unipolar and bipolar, Allegromicrosystems has these hall effect current sensors:
**broken link removed**
Maybe somebody else has a simple way to detect an isolated DC voltage, because I need something like that too, but have been unable to find an inexpensive way to do it ($20 analog isolation amplifiers that need dual supplies or switched transformer setups seem too elaborate for what I realy need it for).
But you could use the analog isolation amplifiers ISO122 or ISO124 from Texas INstruments to measure an isolated DC voltage. But they are $20 and need medium voltage bipolar supplies. THe other way I know of (the much more complicated way) is to make a very very small DC-DC transformer-isolated switching converter that is very low power (since it's really passing a voltage signal and not supplying power) and produces output voltage proportional to input voltage.
EDIT:
Thomas COlins' mention of an optocoupler being used in the analog mode rather than digital got me searching. I found the HCNR200 optocoupler which with a few other parts makes an analog optoisolator whose output is proportional to the input voltage for Much less than $20 and can work with low-voltage single supplies. THanks.
MORE EDIT: IT seems THomas COlins is using the optocoupler in digital mode and not analog mode. But either way, I misinterpeted and it gave me the right idea to lead me along the right path.