I love cloud-based applications, especially free ones. Pixlr is one of my favorites.
Pixlr is a simple photo editor in your browser. To be sure, it isn't nearly as powerful as Photoshop or Photoshop Elements. Then again, it doesn't have the big price tags of those programs. Pixlr is free.
Pixlr is responsive, responds to practically the exact same keyboard shortcuts as Photoshop, and is very easy to use. You can import photos from various sources or start with a blank canvas. It even comes with complex layer support that allows for several blending modes and layer styles. If you don't have Photoshop handy and need to do some image editing in your browser, Pixlr is a really fantastic alternative.
Pixlr works on Windows, Macintosh, and Chromebooks. Pixlr Express is a similar app available for iPhone, iPad, iPod Touch, and Android mobile devices. All are available free of charge.
You can try Pixlr yourself at http://pixlr.com/.