Much depends on why I'm shooting in the first place and what I intend to do with the images afterward. If I'm shooting to address a specific assignment or competition topic, I am going to discard most of the images once I've found "the one." If I'm just shooting casually for myself, I may keep several and stick them in an online gallery. For instance, yesterday I spent the better part of an hour shooting an image to fulfill my online women's photo group's assignment of "Remember When...." Started out with one idea, shot a few images and didn't like what I was seeing, so tried something different, approaching the subject from a different angle and perspective. I knew when I'd shot the "keeper," and so right then took the CF card out of the camera and uploaded the 40-some images into the computer and out of those images, post-processed only two, the one for the assignment and another which was a pretty nice macro. The rest will be deleted.
Another example is the image I showed in the "Photo of the Day" thread. Again I started out with one idea and wound up shooting something very different. In that situation, I had about 4 "keepers," one of which I used to fulfill the word "spring" and another (the one you saw here) I placed in our "Best of the Best" gallery for May.
I NEVER delete images in the camera, even when I can see that they're not so good. Sometimes when you get them in the computer you can find ways to do something creative and interesting with them, perhaps by extreme cropping or inversion of colors, etc. I look at them all in the computer in Aperture, then usually delete at that point, keeping ones which I know I'll be processing and ones I think I might want to experiment with.