Only just found this rather snazzy image editor from Pixlr. It requires that you be connected to the internet, of course, but seems to offer many of the most useful Photoshop functions.
Why would I bother with this when I have Photoshop? Well, on occasion, I will be with a client and want to perform a quick edit of an image for some reason, maybe just to show them how to use the Levels to improve a shot, possibly get rid of something unwanted with the Clone tool, or perhaps add another layer with copyright data. And you ask the question: Don’t suppose you have Photoshop on this machine? Only to receive the usual response about having Microsoft Paint or something else equally dreadful. So, on those occasions, it will be very useful indeed. Or if I bought a Chromebook, I guess. Even works on my Android phone (bit tiny to be usable though!), but you do need Flash installed for it, so may not be for everyone.
Even lets me grab images from URLs rather than the computer, so I can take an image from my website to play around with. Very neat bit of kit!