Suggestions for good lenses for DLSRs

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Hi everyone. I recently bought a Canon 400 D (Digital Rebel xti) kit and my next dream is to buy a nice lens for it. The lens that goes with the kit is ok but I like a better one. Which will you recommend for say, wide-angle lens? What brands outside of Canon is compatible with my camera? Can you recommend a specific brand and model and preferably, not a very expensive one?