The zero-crossing isolator is used for switching AC loads. It's "smoother"... It turns on only when the AC voltage is crossing-zero. Some inductive loads (motors) don't like to be hit with a sudden voltage spike, and the zero-crossing detector prevents this. And, you are less likely to get noise in the AC line. (FYI - SCRs & TRIACs, which are commonly used in dimmers, speed controls, and AC solid-state relays, will turn on any time, but they don't turn-off 'till the voltage goes to zero.)
Light dimmers & motor speed controls need to turn-on at various points in the AC cycle, so in that case a zero-crossing isolator won't work.
I'm trying to pulse 110 volts dc
If you are using DC, there is no zero-crossing and it won't work.
I'm trying to pulse 110 volts dc 3 millisecs apart using an attiny, but im afraid that the emf fluctuations from the coils im driving could ruin my attiny... so I want to use an octocoupler to isolate the voltage spikes, but do not know which one to get.
Well... You can't have 110V going to chip in any case! So, you need to isolate that before worrying about EMF.
Whenever you are switching a coil, you need a protection diode (in this case to protect the optoisolator, or whatever is driving the coil.) About the only time you don't use a protection diode is when you want to use the back-EMF, such as in a switching power supply.
PS. Say I have 2 circuits, one is high voltage and one is low voltage.. and the only line they share is GND.. Is there any chance that the high voltage fluctuations from the high voltage circuit could still affect the lower voltage circuit?
Yes, therte is a chance. But, if you are talking about 110V, it should be isolated anyway (no common ground). I think the general rule is that anything above 48V should be isolated from humans, and that means it should be generally be isolated from low-voltage electronics too.
There (at least) 3 ways to get undesired "coupling" - Capacitively, inductively, and through a ground loop. High frequency and high impedance circuits are more vulnerable than low frequency (or DC) and low impedance circuits.