Canon ST-E2 Speedlite Transmitter

It works a wonder indoors. I rely on ETTL (I will get the hang of manual exposure some day), and I get the same accurate results as with the flash mounted on the camera. Light weight. I never tried it outdoors, but since it is uses IR pulse for communicating with the flash guns, i don't think it would work that great, especially with long ranges (a radio controlled trigger might be a better option).

It is the only way to do wireless ETTL on Canon.
There is a company (forgot the name) that makes a radio ettl trigger but it is basically translates the IR signal to radio signal, you still needs this device for it to work.

It also helps the focus system in low light,