Update:

-Added: Ubuntu Studio, Wings 3D, Krita, Cinelerra
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Most of the GNU Software is found for various platforms (Cross Platforms), MAC, Linux, Windows and some others less known like Solaris, FreeBSD or Sun


-Ubuntu Studio: Let Your Creativity Fly...

My current Operative System, i can say is maybe the most impressive attempt i ever saw for transform an entirely OS into a full-top-high level-art center, a very complete suite with most of the tools listed below perfectly integrated in a Linux Distro, has nice wallpapers, options, look, hell, what can i say, 100% customisable and also supported by canonical, you must give it a try for legion's sake!! (attention, since you are about to install an entirely new OS, i strongly suggest you to read carefully how to do it, also you might like to have your original OS and plus this new one, this is called 'dual boot' read more on google)

-GIMP: GNU Image Manipulation Program

This is what you can call an 'equivalent' of the commercial software Photoshop, since i changed to a Distro of Linux, Mandriva, i've been using this software, and i feel pretty comfortable with it. GIMP also has a great plugin database to enhance it functionality, check the entire site, believe me is wort of it. Also pay attention, GIMP is not better or worst than Photoshop, they are both different but with the same capabilities for those who seek professional working. i'll leave you some examples of what i made with GIMP that i consider my best, i used GIMP since January 2: [link] - [link] - [link]

-BLENDER: The free open source 3D content creation suite

It's a great 3D rendering, animation, modeling tool and it even have it own game engine, that let's you make games (XD), it interface is unique and customizable, it is also tricky but with a little practice you will find is like a candy, personally i prefer Blender to Autodesk Maya Linux, simple by the fact that Blender tends to be agile and professional, but still simple. as GIMP it also have some plugins, there be more around but i haven't searched at all. also a must have. Blender keeps bringing me surprises, i can say that is provably that Blender has the possibility to be even better than anu Autodesk tool

-InkScape: An Open Source vector graphics editor

I also used to handle Adobe Illustrator and i liked it, now i use InkScape, and the description does not lie, is capable of the same that any Corel or Adobe product may offer, stick with this one for your vector based graphics, you will enjoy what can make for you

-Synfig Studio: A tool for producing feature-film quality animation

If you are like me, that enjoy animations as drawing, this tool is for you, easy intuitive and fast, with great effects and customizable interface, it's a great innovation for the few animation programs that does exist nowadays, check the website as most of the GNU programs, you will always find help and tutorials for you to use

-KToon: 2D Animation Toolkit

If you have used Macromedia Flash (Adobe Flash is not like Macromedia Flash) then you will find no problems using Ktoon (and even if you never touched any animation software), the site came online but nothing new is added since a long time, they mention some cool project in the main page, but perhaps that is not taken in work anymore. As i can download Ktoon from my repositories and i'm not sure if the link is working, i'll leave you this link: [link] give it a try, i can guarantee you hours of entertaining animation process (well.... at least i do have them jaja)

-LiVES: Is a Free, Open Source video editor *and* a VJ tool

The best video editing software i've seen, it has copy-paste, frame rate, real time filters and effects, quality setting and many more things that i don't understand since i'm not in the video editing world, pretty exiting anyway.

-LMMS: A free cross-platform alternative to commercial programs

One of the best music production and composition software known around is FL Studio, well, this is what you can call a Linux FL, and is even compatible with the native installation of FL itself, a great tool that you can't see every day, and is X-Platform meaning that even Windows user can taste it, give it a damn try, is and order!.

-Avidemux: A free video editor designed for simple cutting, filtering and encoding tasks

Why should i bother to put a lighter video editor when i wrote about LiVES some lines back? because this one is also a good editor and can be used for fast video editing, light things that doesn't deserve to open LiVES, even tough, still one of the best video editors due to it's functionality and simplicity

-Ardour: Digital Audio Workstation

This software remembers me to a windows's program called 'Live 5' nothing to be with 'LiVES' btw, it's a multi-track song editor/producer, many features, if you keep down with music composition, i demand you to try this one.

-Jokosher: A simple yet powerful multi-track studio

Yep, the description says it all, i have an electric bass, and this software is pretty useful combined with real instruments, even that, you can try to put you own sound files and things, try it out

-Audacity: Is a free, open source software for recording and editing sounds

mm... maybe the best comparison i can make of this software is with Sony Sound Forge, i edited some songs with this program and works good, if you have some mp3 you want to mess with, use Audacity

-K3D: Is free-as-in-freedom 3D modeling and animation software

Pretty descent 3D Editor, i like more Blender, but i tried this one too, not bad tough.

-Wings 3D: A subdivision modeler inspired by Nendo and Mirai from Izware.

Nothing special about this one, i included this program because it has a native compatibility with PovRay Modeler, a 3D scene renderizer with a high level of quality that makes worth to try Wings 3D

-Krita: A graphics application for everyone who wants to get creative with images

This is the true port of Photoshop into GNU/Linux world, the interface and options, all seems to be very like Photoshop, still i preffer GIMP, Krita is part of the free Office Suit for KDE named 'KOffice', a port of Krita for windows is available, tough is not an official port.

-Cinelerra :A video editor and compositor for Linux

One of the best video editors, like LiVES is pretty full of options, plugins, effects and edition tools, i played a bit with it and is pretty fun to work with it, perhaps is a shame that is only available for Linux (not for me, i use Linux)

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About the GNU: General Public License
About the Open Source Software

I also suggest you to look at Source Forge, the life is easy when you know where to look at.