Photoshop (Image Processing)
GIMP (Windows/Mac/Linux)
GIMP is the one that most recommend as a free replacement for Photoshop. It is a free, open source and cross platform tool (Windows/Mac/Linux) with a very active community. It has come a long way from earlier releases, with the option of a tabbed layout window option that looks somewhat like Photoshop, but the user interface is a matter of taste – my opinion is that it still needs work. Photoshop document (PSD) support is limited.
Making GIMP look/work more like Photoshop CS 6:
http://lifehacker.com/how-to-make-the-gimp-work-more-like-photoshop-1551318983
Gimpshop is a version of Gimp that is focused on looking more like Photoshop. Windows only.
Krita (Windows/Mac/Linux)
Free, open source image production software for Windows and Mac. Primarily for drawing and painting, it also has tools for image editing. Nicely done user interface. Can open Photoshop documents.
PixelMator ($29.99) (Mac)
Pixelmator has a fairly simple interface with a good entry level tool set, and the selection tools are decent. Mac only. It understands photoshop files with layers, including text, but layers using blend modes are pre-rendered as previews, so they can’t be later undone in Photoshop, and blend modes from Photoshop layers are not editable in Pixelmator. For some users, this isn’t necessarily a problem. There is also a “Pro” version.
Photoshop Elements ($79.00 educational pricing, bundled with Premiere Elements) (Mac/Windows)
Adobe’s entry level image editor borrows features from Photoshop, but without the recurring licensing cost. It has an “Expert” mode that is like a simplified version of Photoshop, which may be enough for some users. There are some things missing, such as the “Curves” tool. There is a 3rd party add-on that unlocks this feature.
http://www.adobe.com/products/photoshop-elements.html
Elements + is a third party add-on for Elements that “unlocks” hidden features such as the “Curves” Tool
Affinity Photo ($49.99) (Mac/iOS/Windows)
Photoshop replacement which sports a well thought out interface with a rich feature set. It is just similar enough that Photoshop users should be able to pick it up with only a little trouble.
https://affinity.serif.com/en-us/photo/
Acorn ($29.99) (Mac)
Mac only, similar to photoshop and understands Photoshop psd files with layers (including text). While it doesn’t save out layered PSD files, it can save out the layers as PSD files that can then be combined. Very nice uncluttered interface hides some power underneath.
http://www.flyingmeat.com/acorn/
Paint.net (Windows)
Free Photoshop replacement combined with drawing tools, for Windows. Nicely designed. The web site for downloading it is tricky to navigate. It takes some work to get to the actual download links, there are several false links, so be careful.
Photopea (web browser)
Photoshop replacement that runs in a web browser. It is surprisingly complete. It’s ad supported in the non-paid version, but the ads live on one side of the screen and are not annoying.
Photoscape (Mac/Windows)
A “freemium” photo editor which offers basic features in its free form. Missing some crucial features such as level controls – these features are reserved for the pay “Pro” version.
Paintshop Pro ($69.99 or less) (Windows)
Paintshop Pro has been around for many years. It is oriented towards “consumer’ versus “pro” users, but has an extensive feature set. It’s user interface borrows little from Photoshop, so there is some learning curve for Photoshop users. Windows only.
http://www.paintshoppro.com/en/
XNConvert (Windows/Mac/Linux)
Not a Photoshop replacement, it is really a tool oriented for doing batch-based image conversion/manipulation.
http://www.xnview.com/en/xnconvert/
Illustrator (Vector Drawing)
Inkscape
A free cross platform open source vector drawing tool that isn’t super user friendly, but gets the job done.
Open Office Draw (Windows/Mac/Linux)
Simple tool for drawing diagrams and other vector based work. Cross Platform (Windows, Mac, Linux)
https://www.openoffice.org/product/draw.html
Affinity Designer ($49.99) (Mac/Windows)
Tool that rivals Illustrator in some of its capabilities. It has a polished interface that Illustrator users will be able to pick up fairly quickly. Understands Illustrator and Photoshop files.
https://affinity.serif.com/en-gb/
Indesign
Scribus
A free, open source tool for desktop publishing and layout. It isn’t as nice as Indesign, but is still capable for basic layout jobs.
http://wiki.scribus.net/canvas/Scribus
Affinity Publisher (Mac/Windows) ($49.99)
Nicely designed replacement for Indesign. Indesign users should be able to pick it up with little difficulty.
https://affinity.serif.com/en-gb/publisher/
Lightroom (Photography Management and Touch Up)
Adobe’s Lightroom is part of the Creative Cloud Suite, but is also available as a standalone product, one of the few pieces of software in the Creative Cloud suite that is. Lightroom 6 is the last version that will be a standalone product, and support for it will be discontinued at the end of 2017. From then on, the only option will be Lightroom CC.
Lightzone (Mac/Windows/Linux)
http://www.lightzoneproject.org/
Free cross platform photo manipulation and management software for Windows, Mac and Linux. Requires registration for download.
Darktable (Mac/Windows/Linux)
Free Lightroom replacement with a decent user interface. It handles all basic tasks such as photo processing and simple image management.
Photos (Mac)
http://www.apple.com/osx/photos/
Apple’s photo management software for the Mac. Basic image processing tools, but a decent retouch tool. Not really a replacement for Lightroom, but may be enough for casual users.
Google Photos
Google-Centric web-based tool for managing, cataloging and editing photos. Replaces Picasa.
Exposure X began as an application for photo manipulation – notably has extensive film looks (with descriptive designations like Fuji Velvia etc) and editing/retouching, but has grown into a photo management application as well. Educational pricing is available.
http://www.alienskin.com/exposure/
Aftershot (Mac/Windows)
http://www.aftershotpro.com/en/
Aftershot is an image editor and cataloger which is frequently discounted. It offers similar features as Lightroom, and has a “send to external editor” function that can send uncompressed TIFF images to Photoshop for further work.