I’ve used Photoshop and Photoshop Elements, and I’ve used The Gimp on Linux and a bunch of other programs, and for big changes they are still the best. If all you want to do is crop and lighten and sharpen, however — which for the vast majority of photos is all I really want — then an online tool like Picnik works just fine.
So far I’ve tried it out on a few pictures and it worked great, although saving the photo after I was done took awhile (as MG Siegler also notes at ParisLemon). Still, a nice addition to Flickr’s tools. Update: Someone from Box.net emailed me to let me know that Picnik is also integrated with their online storage solution, so you can edit any photos that are stored on Box as well.