Download Gimp
GIMP (the GNU Image Manipulation Program) first appeared way back in 1996, and has been regularly updated ever since, so it's no surprise that the package is the most powerful free photo editor that you'll find anywhere.

GIMP

There are tools to correct colours, enhance contrast, brightness and more; sharpen or blur an image, fix perspective problems, remove red-eye; add special lighting effects, turn a photo into an oil painting, and create an animation.
The paint tools alone are amazing, there's full layer support and lots of ways to extend the program.
All this power does mean GIMP takes a while to learn, but don't let that put you off - if you've any previous image editing experience then you'll be doing useful work within minutes.


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2. Paint.NET

Download Pain.NET
It may not have anything like the power of GIMP, but there's still plenty to like about Paint.NET. It's strong on image editing basics, with options to resize and rotate your photos, a good range of selection and paint tools, and some excellent special effects.

PaintNet

Support for layers means you can apply your edits to part of an image only, and the program can be extended with plugins.
Best of all, though, Paint.NET's clear and straightforward interface means it's very easy to use, even if you're a complete graphics novice.


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3. PhoXo

Download Phoxo

PhoXo

PhoXo has been around for almost 10 years now, and it's grown into a very useful editor with something for everyone. Kids can play around with the clipart, customising images with cartoon cats; practical types will enjoy options like the ability to add a custom watermark to an entire folder of images; and everyone else will appreciate the drawing and paint tools, image transformations, effects and more.
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4. Funny Photo Maker

Download Funny Photo Maker
Funny Photo Maker has no layer support, no smart selection tools, no paint or drawing options. But that doesn't matter, because what it does have is an amazing range of effects which you can apply in seconds.

Funny Photo Maker

So you can add someone's face to a movie poster, magazine cover or a dollar bill; transform a still image into an animation; add rain or snow to a photo, apply some excellent frames, and turn your photos into a collage, amongst many other options.
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5. Photo Pos Pro

Download Photo Pos Pro
Its tiny toolbars and dated interface mean Photo Pos Prodoesn't look too promising, but begin to explore and you'll soon be impressed.
There are plenty of selection options, a wide range of paint tools, colour corrections and more
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Photo Pos Pro Photo Editor

You get reasonable support for layers and masks, lots of effects (all extremely configurable), and a scripting tool which helps you automate many editing tasks.
And the program offers some fun touches, too, like a set of "Magical" effects which help you add fireworks, bubbles, stardust or similar decorations to an image.
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6. PhotoScape

Download PhotoScape
PhotoScape won't appeal to editing experts - there's no layer support, for instance - but if you're just looking to have some quick fun with a photo then it's a very different story.
There are hundreds of clipart images ready to paste into your picture, for instance. You can add custom speech bubbles to an image, apply some great special effects, even create animated GIFs.


PhotoScape

Most of these options are surprisingly configurable too. So while other tools might just have a single "turn this photo into an oil painting", for instance, PhotoScape supports 11 artistic styles (Pastel, Pencil, Cartoon, more), each of which can then be customised further.
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