Welcome to the forum.
I had the Canon EF-S 10-22mm and it was an outstanding 'ultra wide angle' lens. It's a bit expensive...and the Sigma 10-20mm has a good reputation and a lower price.
Why are you so dissatisfied with the 17-40mm F4 L? Sure, it's a wide angle lens, but it's designed to be a wide angle lens for a full frame (or 35mm film) camera. On a camera like the 50D, the smaller sensor means that it's simply not a wide angle lens. But otherwise, it's a Canon L lens, which means that it's top of the line.
A good 'standard' zoom, at a good price, would be the Tamron 17-50mm F2.8 (non VC). I had this lens and loved it. Especially because it was less than half the price of the Canon EF-S 17-55mm F2.8 IS. If you've got the money, I'd recommend the Canon, but the Tamron is really good for the price. They have a new version, with VC (their version of IS) but it's more expensive. Sigma has a version as well.