GIMP

• Popular among open-source enthusiasts, GIMP offers most features of professional graphics editors for free. These include basic features such as cloning and filters, but also has full support for layers and channels. These more advanced tools are useful for creating professional-looking scrapbooks. GIMP is available on multiple platforms, including Linux, Mac OS X and Windows 7. The interface is visually similar to commercial competition such as Photoshop and Paint Shop Pro, but several key differences -- such as how the selection tool works -- can make it a bit difficult to pick up.
  
  

Photoscape

• Simpler than GIMP but also easier to use, Photoscape offers a wide variety of filters as well as standard options such as cropping, noise removal and caption adding. Using Photoscape, you can also add digital frames to your pictures or create animated GIFs. Finally, this program also supports batch processing, including the rapid renaming of image files and the application of filters. It is a Windows program.
  
  

Paint.NET

• Originally intended as a replacement for Windows' built-in Paint program, Paint.NET outgrew its humble origins and now stands on its own as a lightweight pixel-based image editor that is easy to learn and use. It does not offer dodge or burn tools for advanced editing, but does feature simple filters such as red-eye removal and a customizable interface. Finally, it has a light footprint in terms of processor and memory usage, making it ideal for netbooks and other lower-performance computers with weak graphics cards.
  
  

Pixia

• For image editors looking to edit digital illustrations as well as digital photographs, Pixia offers the best of both worlds. In addition to the wide variety of filters and color adjustment options, the program supports multiple layers as well as filter masks, which allows the editor or artist to apply a filter without permanently altering the image. Pixia does not have native support for GIF files, so a free converter such as XnView may be needed to edit GIF files in Pixia. Help documentation is available from the website, but not packaged with the program. It runs in Windows XP, Vista and 7.
  
  

PhotoFiltre

• Great for beginners, PhotoFiltre offers a full package of features with a simple and intuitive program interface. As a lightweight editing tool, it does not have all of the features of other photo editors, such as the ability to create and manipulate image layers, but it is still capable of image-editing standbys such as contrast adjustment and the magic wand tool. It is available for Windows 98 and up.