The best 2 ultra wide are
The Canon - EF-S 10-22mm f3.5-4.5 USM - Link
and the cheaper
Sigma - 10-20mm f/4-5.6 EX DC HSM - Link
Tamron and Tonika also do zoom lenses in that range but they don't get such good reviews.
You could also buy a prime but they aren't any cheaper and you loose the wider range. You also have to remember the crop factor of the 400D (1.6) so these are actually 16-32 and 16-35 respectively.
I hope that helps
Cheers
Colin
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