I see Flickr as the "MySpace" of photography. Whether you like it or not it's been around a long time and it has a lot of people signed up. It's sort of the place where a lot of people "hang out" and discuss and share images.

DeviantArt seems to be very focussed around imagery of all kinds that can be very artistic - almost to the extreme - and seems to appeal mostly to "non-conformists". It's Flickr for people who think Flickr is too mainstream for them.

Not familiar with ImageKind.

I've been to SmugMug before but I don't stay. Generally I find it seems to be a great place for people who want to share photos with family and friends online and at the same time use it as a backup service.

PicasaWeb fits here too. It's much like SmugMug to me though not nearly as mature and still suffering from lack of features. It's tied into the Picasa desktop app very well which makes it VERY easy to share photos with family and friends so it has it's place.

That's my