What would be good a baseline E TTL-II flash for a Rebel XT?

[RayFromAlaska]

Such a question has probably been beat to death in this forum, but I am new to digital photography, have decided to buy a Canon Rebel XT with the stock lens, realize that some Canon flashes are very expensive, and have no idea if other than Canon flashes would be 100% compatible with the Rebel XT. This camera is designed to work best with E TTL-II flashes. Are there any flashes other than Canon's that would be just as compatible?

I plan to use the camera's flash, but I have read in some of the professional reviews on the Rebel XT that an external (mounted) flash helps greatly with camera focusing (?). What I am trying to do now is to gather all the information I can in relation to flashes, including Canon's, that are 100% compatible with the Rebel XT, and of a fair quality, but not expensive. I will only be using the "mounted" flash every now and then, mostly indoors. I am not a professional photographer, but enjoy taking pictures whenever I can for fun.

I would appreciate any comments or experiences you may have on this subject.

Thanks.