my 15mm doesn't vignette but it's a fisheye. canon makes a 14 f/2.8 as well but i think that's the widest. and pricey. nikon has a 14-24 ultrawide which works great on a fullframe. canon's offering is a bit longer, 16-35.
Really wide zooms for full frame are pretty hard to make. There's really only one good one, and it doesn't take filters (which worries on me a wide angle, since I like to have the camera close to the things I'm shooing with it..) and it's a Nikon and it's terribly expensive.
I'd sit tight and get the 16-35 or the 17-40 if you're on a budget...
for canon lenses made for crops (EF-S), that's true. but the 'digital' lenses made by 3rd parties doesn't have that physical problem. i tried out my sigma 30 and tokina 12-24 on my 5D before selling them, and they fit just fine but had a ton of vignetting.