In reply to Robert Durran:
I've wondered this myself, but never really looked carefully.
I've just opened one of my photos on UKC, Flickr and from file in Windows Picture viewer or whatever it is now, and compared them on the same laptop screen. If I'm honest, I can't tell the difference if I look carefully. The initial impression is that the Flickr one has most saturation, but actually, I'm 99% sure that's because its the only one that opens on a dark background rather than a light.