Despite its somewhat unfortunate name, GIMP (or GNU Image Manipulation Program), is one of the more comprehensive, and free, alternatives to Photoshop; you can access many Photoshop-like features, from masking to layering and plug-ins. The interface for GIMP is also relatively easy to use.
This program is mainly distinguished by offering vector support over bitmaps, meaning that it can enable more sophisticated editing than many other programs. Inkscape is also worth investigating for its flexibility, with the program available on different operating systems.
Unfortunately only available for Windows at the moment, Pixia has been popular as an image editing program in Japan for some time; its interface is basic but accessible, and the program works particularly well if you want to combine design work with a stylus and digitization.
A web based program, Splashup is open source and focused on providing pixel control and a lot of different options for depth changes – the interface is suitable for beginners, making Splashup a decent choice if you want to edit photos online.
A basic Google product that can be easily downloaded or found with operating systems, Picasa won’t give you the depth of features that you get with Photoshop, but can still be used to correct photographs, and to catalogue different shots.
For Macs only, this program is open source and mainly designed for use with OS X Cocoa – Seashore can handle text editing and different layers, as well as alpha channels, and is similar to GIMP in terms of its native file formats; this means that you can use both in conjunction with each other.
Mostly a photo enhancing program, Photoplus is only available for Windows, and should be viewed as a simple, but effective tool for fixing up any images that you have on your computer.
Free with Mac devices, Mac Preview actually has a lot more to it than is first apparent – while the program is mostly used for opening images and PDFs, you can do a surprising amount with filters and image editing once you get into the program.
A modified version of the old Windows favourite MS Paint, Paint.Net is open source, and designed to feature many Photoshop-lite features, from filters to text editing and masking.
Adobe Photoshop Express
If you really can’t bear to give up on Photoshop, Adobe do provide a basic browser based version of the program, with files hosted on cloud servers; you get a lot of the most essential Photoshop tools, making it best for simple editing and beginners who are intimidated by the sheer scale of options available for the main version.