28mm isn't very wide on that camera...there are basically two steps wider for you. The first step would be to the 'normal' range on these digital bodies...this starts at 17 or 18mm. The cheapest lens is the 'kit' lens that usually comes with the camera...EF-S 18-55mm F3.5-5.6. The best choice is the EF 16-35mm F2.8 L or the EF-S 17-55mm F2.8 IS. There are plenty of lenses in between...the Tamron 17-50 F2.8, for example.
The next step wider is really wide. The Canon EF-S 10-22mm F3.5-4.5 is the best choice here.
The 17-40mm F4 L and the 16-35mm F2.8 L are not EF-S lenses but would be good wide lenses for the 30D. They are more expensive than comparable EF-S lenses though...although the EF-S 17-50 F2.8 IS, is quite expensive. The widest lens, the 10-22mm is an EF-S lens though.