GIMP (GNU image manipulation program), is arguably the best free image editor available. It offers many of the same features without Photoshop's considerable cost. However, you'll have to plan on spending some time learning the ins and outs before feeling entirely comfortable with using GIMP because it's not very intuitive.
It's complicated. Users familiar with Photoshop will notice a similar feel, but if you're not much of a techie, you might find GIMP frustrating. Newbies might easily get lost in the shuffle. There isn't a lot of support out there, other than some user forums, and what tutorials and training materials do exist can be inconsistent at best. Upgrades and new features don't manifest on any set timetable. GIMP supports many file formats and works with almost any operating system.
Plays the field. GIMP gives you many of the same dazzling features as Photoshop: filters, brushes, text tools, layers, basic editing and color correction, and effects. It's not quite as slick but can do the job nicely. However, when it comes to really advanced features, they won't be there for you. Plug-ins will give you more options (content aware tools, support for RAW), but you'll need to scour the web scene for those. GIMP does not provide any tools for organization or management of your collection, so you'll need to house your files elsewhere.
Works well, with effort from you. Despite having many of the same features as Photoshop, GIMP does not use nearly as much memory. GIMP also runs on almost any platform, including Linux. The editing layout can be confusing, especially when you have more than one image open at a time. GIMP supports layers to a degree, unlike some other free image-editing programs like Google Picasa. The cloning and healing tools work well but are not as graceful as Photoshop's. You can save images in nearly any file type. GIMP does what is expected, but if you want a lot out of it, you'll need to work for it.
1. PC Advisor
GIMP is rated 3 out of 5 stars overall by this reviewer, broken into subcategories: 2 out of 5 for ease of use and 3 out of 5 for features. The author provides an insightful review of GIMP's features and usability. The reviewer says GIMP offers all the core tools you'll need, though it has some shortcomings.
Review: GIMP 2.6 Review, Paul Monckton, May 18, 2011
2. PC World
GIMP gets rated 4 out of 5 stars here. The reviewer praises GIMP for not following trends to make things look pretty, but for focusing on function.
Review: Open-source Freeware Fave GIMP Ready to Take on Photoshop, Clare Brandt, March 13, 2012
Sue Chastain, About.com's guide to graphic software, provides a good list of pros and cons for using GIMP, a free web-based photo-editing program, but offers no specific testing or comparisons.
Review: The GIMP Review, Sue Chastain, Not Dated
CNET editors rate GIMP 5 out of 5 stars. The author regards the software as comparable to Photoshop, citing several options common to both.
Review: GIMP, Seth Rosenblatt, Oct. 6, 2008
This is a very brief overview of GIMP, along with nine other comparable photo-editing software packages, some free, some not.
Review: 10 Photo Editing Programs (that Aren't Photoshop), Tim Moynihan, May 17, 2013
Enid Burns provides a quick snapshot of five free photo-editing resources, including GIMP. She says that GIMP is the "closest to Photoshop" in the roundup.
Review: Before You Resort to MS Paint or Piracy, Try the 5 Best Free Image-Editing Tools, Enid Burns, Feb. 3, 2013
GIMP is compared to Adobe Photoshop CS5 Extended, Paint.net v3.5.5 and Google Picasa 3.9. Each is tested on interface, import and organization, editing, and sharing.
Review: GIMP 2.6.1, Jeffrey L. Wilson, Oct. 7, 2010
Mike Williams surveys 10 free image editors, varying in degrees of complexity. GIMP is ranked at the top of the list.
Review: Best Free Photo Editing Software: 10 Top Image Editors You Should Try, Mike Williams, March 5, 2013
9. What Digital Camera
This reviewer looks at free photo-editing software, ranking GIMP 2.6 the best over Google Picasa 3.9, RawTherapee 3, Photoshop Express, PhotoPlus Starter Edition, Fotoflexer, Photoscape, Seashore and Pixlr.
Review: Free Photo Editing Software, Matt Tuffin, Feb. 15, 2012
Photo-Tips-Online.com conducts a high-level comparison of GIMP, Adobe Photoshop, Adobe Lightroom, Apple Aperture and Corel PaintShop Photo Pro. Reviewers suggested spending time with each program, as all are worthy, but it comes down to individual preferences.
Review: Top 5 Photo Editing Software Programs, Editors of Photo-Tips-Online.com, April 11, 2012