GIMP (Mac, Linux and Windows)
GIMP is one the most versatile graphics manipulation tools. It has some of the most amazing image manipulation tools such as Layers, Brush, Cropping and other tools and plugins similar to Adobe Photoshop which is the commercial leader. Read more about GIMPs features here.
MyPaint (Mac, Linux and Windows)
MyPaint is a cool digital painting app. It is simple, fast and lets you focus on the art rather than the software. Download it here.
Krita (Linux, Windows)
Krita, the full-featured painting application for digital artists with natural looking brushes and other tools.
Darktable (Mac, Linux)
Darktable lets you process your Raw and JPG images. It has some of the advanced image processing features such as color correction, tone correction etc. Detailed feature list is here.
Inkscape (Windows, Linux, Mac)
Very good vector illustration tool similar to Adobe Illustrator or CorelDRAW. Free and very capable tool for designers. It is very easy to edit nodes, perform complex path operations, trace bitmaps and much more. Inkscape has an aim to maintain a thriving user and developer community by using open, community-oriented development.
Calligra Flow (Linux)
Calligra Flow is great diagramming tool similar to Microsoft Visio. Free and great to use. It can open Visio files too. Check it out here.
Scribus (Windows, Linux and Mac)
Scribus is an Open Source program that brings professional page layout to Linux/UNIX, Mac OS X, OS/2 Warp 4 and Windows desktops with a combination of press-ready output and new approaches to page design. Scribus is an alternative to Microsoft Publisher, Adobe InDesign, FrameMaker etc.
Audacity is the best sound editing and recording software tools out there. Highly capable, downloaded and recommended.
Hydrogen is an advanced drum machine for Linux. It’s main goal is to bring professional yet simple and intuitive pattern-based drum programming.
Mixxx (Mac, Windows and Linux)
Mixxx is a top notch DJ software, has everything you need to start making DJ mixes in a tight, integrated package.
Blender (Windows, Linux and Mac)
Blender is the free open source 3D content creation suite, available for all major operating systems. Blender has some of the advanced modeling features and it is commercial grade.
Synfig Studio (Linux, Mac and Windows)
Synfig Studio is a free and open-source 2D animation software, designed as powerful industrial-strength solution for creating film-quality animation using a vector and bitmap artwork.
Pitivi is a free, intuitive and feature rich movie editor for the Linux desktop
Kdenlive (Linux, Mac)
Kdenlive is afree open source video editor.
Sparkleshare (Linux, Mac and Windows)
SparkleShare is an alternative to the popular DropBox, Google Drive kind of file sharing services, free and open source. You can setup where to sync etc.