Pixlr Editor is the most popular advanced online photo editor. No download is required, and it's 100% free.

If you're used to working with Gimp, PaintShop Pro, or Photoshop you will feel right at home with this online image editor. It contains a lot of the features that you normally see in desktop graphic design applications. Jump in and start using this tool with no registration required.

• No account necessary. Create an account for free storage space if you prefer. 
• Nothing special to download or install. Just fire it up in your browser when you need it.  
• Provides tools like layers, lasso tool, brush controls, cloning, and filters.
• Optional private Pixlr Library where you can store all your photos and images. 

Tools include red-eye reduction, a spot healing tool, drawing tools, clone tools, sharpen, blur, and many more. 

Filters include mimic HDR, glamour glow, tilt-shift, Gaussian blur, vignetting, noise, and many others.

Adjustments include advanced concepts like Levels, Curves, Cross Process, Desaturate, Auto Levels, Hue/Saturation and Brightness/Contrast.