The Yungnuo YN560ex is a very inexpensive flash that can be used as an off-camera E-TTL Slave (or manual) using both Nikon and Canon's infrared wireless systems.
Here are some YouTubes to help guide you with the setup:
Nikon using PopUp Flash as Controller:
Using Canon System with PopUp Flash as controller:
Nikon using SB series flash in hotshoe: