Why do you use online photo editors like picnik.com instead of desktop software?
Note: Google have recently announced that as of April 19, 2012, picnik will be discontinued.
I used Picnik several times when sharing stuff on flickr, mostly just for cropping something that I didn't see before uploading. It also does (reasonably) good white balance correction. Its convenient and easy enough to use.
I'm a SOOC (straight out of camera) type of person, though, so I don't do much post-processing. If I can't fix it with a crop or WB correction, I generally go back and re-shoot it.
I have used Picnik for some basic edits of pictures I've upload to Flickr. It is okay for a quick crop or levels correction of the mobile phones I upload.
One things I've noticed about the free version at least is that when you save the image the JPEG compression is quite high, which is good if you want to reduce the file size but bad if you want to keep the quality of the original. So my advice would to always, save it as a new image and keep the unaltered original.
Picnik is integrated into SmugMug, so I've used them just because it was easily available when I was away from home and my real tools. At home, I'd never dream of using a slow web based service over my native tools on my mac when I have them available.
I've mostly used it to make simple tweaks to images I've uploaded from my phone while away from home. Stuff like cropping a shot tighter to remove someone from the background, or fixing the wild color cast because the iPhone got confused by the neon behind us. I'd love to do that on the phone, but have yet to find a decent tool to do it with.
I have not seen anything quite like picnik.com. online. Photoshop.com is close enough. But picnik is the best of the lot.
One interesting use of online photo editors is that, you can edit anywhere, from your work or from your computer, or school, or even a library. So that is the greatest thing about that.
Also you can use it kind of a temporary store in cloud. picnik lets you store upto 100 photos in its premium version, all with the history of edits.
Personally I don't like online photo editors, because I can't save the presets like Ligthroom, the quality is very low in comparation with software for edition like Photoshop or Ligthroom, or for example Gimp (open source).
Personally I preffer quality over quickly or easy of use.
Ok, I do not use these tools. Maybe they are good for very quick purposes and in cases you cannot use other SW.
However, for me is much comfortable to carry GIMP Portable with me on a flash disk - with all known tools, plugins and presets. For much easier things like quick conversion and cropping I also use IrfanView, so I do not have any reason to use online tools.