As with all lens choices it depends what you want to shoot with it, and how.

Some very good lenses mentioned and they all have different features and strengths, if they suit you, and the type of shot you want to make is another question.

The Canon 17-40 that you mentioned is a great lens and has a reputation for quality.  I would say it is more of a general walk about lens than a specific wide angle lens when used on a camera like the 400d with it's 1.6 crop factor.  It has the advantage of fitting all Canon cameras if you should ever upgrade to one with a larger sensor, and as Danny has pointed out is F4 at all focal lengths. 

The Sigma 18-50 also has a good reputation and is F2.8 all the way this may an advantage in some situations eg indoors or poor light.  It is slightly less money than the Canon but probably not as weather proof and will only fit cameras with a smaller sensor eg 400d, 30d etc. Again more a walk about lens than a specific wide angle lens.

The Sigma 10-20 is a true wide angle lens on a crop sensor camera. Consider carefully though if you want a specific wide angle lens. It is a specialist, and therefore requires specialist technique to get the best out of it.   Results from this lens can be stunning if this is the type of shot you are after, it is in my opinion the pick of the wide angle lenses available for your camera the quality is second to none.  The fact that is F5.6 at 20mm is of no consequence to most users as they would be using a much smaller aperture  in practice when shooting wide angle landscapes. 

And every one said, 'If we only live,
We too will go to sea in a Sieve,---
To the hills of the Chankly Bore!'
Far and few, far and few, Are the lands where the Jumblies live;
Their heads are green, and their hands are blue, And they went to sea in a Sieve.

Edward Lear
http://www.nonsenselit.org/Lear/ns/jumblies.html