I just want to clarify something that I believe to be true but I am not great with lighting so I am not sure. I have a Canon 580EX II that I have used on the hot shoe directly of a Canon 20D. I want to experiment with off camera flash more and I have a TTL cord that I found made by Promaster. Will this work for ETTL, and what other options are there for using ETTL for this flash off camera? I believe with the exception of the new flex PW's the other wireless systems will not work with ETTL - correct? But I may be able to trigger it optically? Thanks!
Yes, you can use the TTL cord to do everything you do as though the flash is in the hotshoe. However, you will be very limited to about 2' with the cord unless you DIY extension. With another flash acting as Master on camera to Slave a flash off camera you have ETTL too. However, this has limitations that many do not care for.
You can't trigger it optically without an additional optical slave trigger device because the Canon flashes do not have this built in.
The new PW TT5 and TT1 can do ETTL. RadioPoppers are getting mostly positive reviews (do a search on here to learn more). There are no other radio triggers that I am aware of that do ETTL.
Check out Chuck's site for more info on triggering your flash. Though he does not go into radio slaves you can get an understanding of the use of the Canon flashes.
