Hi,
If it's just a signal, and the grounds are connected, sometimes you can just use a capacitor and series resistor and that provides enough isolation.
There's a trick you can use with AC but not with DC because AC doesnt need a directly connected ground...
If the grounds are not connected, sometimes you can use two sets of capacitors with series resistors, one set in the signal lead and the other in the ground lead. This isolated the signal and the ground lead too, but you have to be careful when connecting circuits that have very different ground potentials because a higher initial current surge can occur. With the resistor values chosen appropriately, you can minimize this problem.
If you want to you can also incorporate protections circuits consisting of zener diodes to protect against a current surge that could bother the main supply source.
You didnt say how much power you need to transfer so i assumed you were talking about signals only.