Picnik - This is probably the cleanest design of all of the online editors. Each tools are accessed through tabs, and this presents both a benefit and a drawback. Without multiple tools on the screen, it's not as easy to switch between tools. That said, you always know what too you're working with, making for a very straightforward editor.
Splashup - Still in beta, this Java application has more functionality than Picnik, treading close that of a standalone photo editor. One thing that was noticeably missing in my test (and a must, in my opinion, for any editor that allows selection of parts of images) is the "magic lasso" tool. Still, it's impressive how much you can do beyond adjust color and tone, and the ability to work in layers is invaluable.
Fotoflexer - Another editor with tools laid out in tabs, Fotoflexer offers the most options for direct photo sharing. Everything from Facebook and Flickr to Orkut and Twitter is available. Layers are also available to work in, though not to the extent that you can in Splashup. This editor does have no features in beta that are worth testing, including "Curves," giving you access to the histogram.
Pixenate - Even though it's one of my favorite editors to play with, don't mistake that for not having options. Pixenate has the friendly tool options you expect (including a "spirit level" which comes in handier than you'd think!), as well as a toolbox of fun effects. I haven't seen another editor that allows you to take one photo and make an Andy Warhol-style pop art version of your image.
Photoshop.com - You would think this would be the granddaddy of them all, but it's not. Originating as Photoshop Express back in March, it's matured beyond that beta version. It combines the essential editing tools with photo sharing capabilities, and also makes for clean distribution to other Web sites. I really like the layout and having my online editing and sharing in one spot.