Some objections have been raised to the listing linked in the previous post. So what are the best photo sharing sites? Chris S. nominates altphotos.com, for instance, and Howard French obviously likes flickr (I don't, so much, because I dislike the everything-oversharpened-all-the-time look).
I typically visit Trek Earth, because at least it's organized around identifiable places—the tendency of most online photographers to not identify, and thus seemingly not to even care about, where on Earth their pictures were taken drives me insane—and pbase and PhotoSig because I'm a lens nut and those sites identify pictures by the lenses used to make them.
Any other good ones? Reasons?