Funny, before I saw this thread, I posted an answer to another post here, and I pimped the Sigma 28mm:
On a Nikon body (D300) I use the Sigma 28mm f/1.8 EX DG Aspherical Macro. It's a great walk-around lens, and I really dig the macro feature. The lens is freaky sharp and really nice in low light (low light capability is important to me, may not be to you). I love to use in indoors at parties and the like - the focal length is good for that, in that you take in more of the subject and it gets more depth of field compared to a 50mm, the lens is very fast (f/1.8) so I can often shoot in low indoor light without flash. But it's also a great outdoor lens - the wideish focal length is nice for both architecture in the city and for landscapes. And it's an affordable $250-$290. The downside to this lens is that it is pretty big and heavy for a prime comparatively speaking (the 50 is much tinier), and the auto focus is noisy... but then again, it's solidly built and the big glass results in better IQ. As usual with Sigma, they make this in a Canon mount, though I've not read reviews of how this lens does on Canon bodies. On the Nikon, I love it. You may also look into the the 24mm version. Oh, one other thing - if you ever want to go FX, you'll still be able to use this lens, which is nice... after all, you may buy a disposable body, but lenses are to be collected!