I agree with PWP that you should skip optical and go straight to radio. They're generally more reliable, and their range is typically farther than optical systems.
I personally recommend the Phottix Strato II
transmitters/receivers. They're about $100 for a trans/recv pair
, and each receiver is around $60.
They have a fantastic range (advertised 150 meters, but I found 100-120 meters were the limit of 100% reliability) and they have four channels and four groups. The group feature is very neat - the transmitter has A,B,C,D buttons and each receiver has a group selector switch. You can quickly and easily turn on and off any combination of groups. I use this during wedding receptions with several flashes on lightstands to decide moment by moment which flashes I want to fire.
If you ever need to remotely fire your camera, the receivers come with this ability built-in (even comes with the camera cable).
Also, the transmitter has an on-camera E-TTL pass-through. This lets you use a E-TTL flash on your camera with the transmitter on or off. Very handy.
Okay, end of sales pitch.EDIT: Just read PWP's link to the YN-622C review.. those sound pretty awesome. Definitely look into them. E-TTL radio remote for a little less than the Strato II's? Amazing.