GIMP stands for GNU Image Manipulation Program. It is a free software for photo retouching, image composition and authoring. It can act as a simple paint application and at the same time expert quality photo retouching software.
It can do anything with plug-ins and extensions.
GIMP is the biggest source of confrontation ahead very costly Adobe Photoshop. It has all the features that are essential for head to head comparison with Photoshop.
It has all painting tools including Brush, Pencil, Airbrush, Clone, etc. It has very powerful gradient editor and blend tool with support of custom brushes and patterns.
It has also Advanced Manipulation options like full alpha channel support, layers and channels(You must’ve known that if you have used Photoshop),Undo/Redo options,text layers, transformation tools like rotate, scale, shear and flip, selection tools like rectangle, rounded rectangle, ellipse, free, fuzzy.
Now something which is unusual in Photoshop, it contains GIMP animation package which consists of hell lot of options.
Don’t you think that these endless features are enough to prompt you to try this once. Also due to open source backup team numbers of new features are added to this time by time.
GIMP is distributed with almost all Linux packages, also it has support over Windows and MAC.

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2) GIMPShop:

It is modified version of GIMP designed to replicate the Adobe Photoshop. It is intended for the open source users which have been using Adobe Photoshop and just have shifted to open source. It has very similar interface to the users as compared to photoshop. Like GIMP it is very portable and supports almost all platforms.

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3)Krita:

It is just another Image Editing software like above based on KDE. It is generally distributed with Koffice. It is enormously flexible which gives full scope to your creativity relating to images.
Krita is rally fun. It has very easy-to-use features like guided painting, and advanced features like colorspaces, layer groups or tonemapping. Krita is free software so is improving day by day.

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4) Panorama Tools:

It has multiple utilities like PTOptimizer, for optimizing control points, Panoinfo, PTmender, PTblender, PTtiff2psd and PTuncrop ion.
The Panorama has Gimp plug-ins. The gimp-plugins are rearranged and updated regularly. A fully 16bit color depth compatible version of the Panorama Tools Photoshop plugins.

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5)Hugin

It is very very interesting Image stitching software based on Panorama. It can stitch any set of overlapping pictures. Use this software some of the greatest mosaics. Softwares like Hugin are few in numbers.
You can take multiple images from different angles and stitch them into a beautiful panoramic view.
You must try this.

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Click here to see one mosaic effect of some simple pics.