I have just been using free google chrome browser apps.
Polar is fantastically easy to to and powerful for tweaking colors and such... Its just has a few querks but no deal breakers for me. I should mention it doesn't have a raw mode though unfortunately.
In the rare chance I need to do some editing to remove annoying things, I used Pixlr... Pretty much a Photoshop clone like Gimp, but again a free chrome app.