Drupal | open source community & content management platform

Drupal is an open source content management platform powering millions of websites and applications. It’s built, used, and supported by an active and diverse community of people around the world.
Use Drupal to build everything from personal blogs to enterprise applications. Thousands of add-on modules and designs let you build any site you can imagine.
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Drupal's Video Module (with a few other tweaks) turns Drupal into something approaching Youtube as a video sharing site.

Clementine | free music and audio file player

"Clementine is a modern music player and library organizer. It's a multiplatform music player, inspired by Amarok 1.4, focusing on a fast and easy-to-use interface for searching and playing your music.

Features:

• Search and play your local music library.
• Listen to internet radio from Spotify, Grooveshark, Last.fm, SomaFM,Magnatune, Jamendo, SKY.fm, Digitally Imported, JAZZRADIO.com,Soundcloud, and Icecast.
• Search and play songs you've uploaded to Google Drive.
• Create smart playlists and dynamic playlists.
• Tabbed playlists, import and export M3U, XSPF, PLS and ASX.
• CUE sheet support.
• Play audio CDs."

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Pinta | open source paint program

Pinta is a free, open source drawing/editing program modeled after Paint.NET. Its goal is to provide users with a simple yet powerful way to draw and manipulate images on Linux, Mac, and Windows.

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Calibre | ebook management software

calibre is a free and open source e-book library management application developed by users of e-books for users of e-books. It has a cornucopia of features divided into the following main categories:

• Library Management E-book conversion 
• Syncing to e-book reader devices 
• Downloading news from the web and converting it into e-book form 
• Comprehensive e-book viewer 
• Content server for online access to your book collection

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UFRaw | raw image editor

"UFRaw (which stands for Unidentified Flying Raw) is an application which can read and manipulate photographs in raw image formats, as created by many digital cameras. UFRaw used to be available both as a stand-alone program and as a GIMP plugin. However, since UFRaw-0.19, the GIMP plugin is no longer officially supported. As a stand-alone program, UFRaw can be invoked with a graphical interface, or as a command line batch processing utility.[1] UFRaw reads raw images, using dcraw as a back end, and supports color management via LittleCMS, allowing the user to apply input, output, and display color profiles (see also Linux color management). Thanks to dcraw's versatility, UFRaw supports nearly all of the numerous raw image formats used by digital camera manufacturers." [1]
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Dia | Diagram editor

Dia is roughly inspired by the commercial Windows program 'Visio,' though more geared towards informal diagrams for casual use. It can be used to draw many different kinds of diagrams. It currently has special objects to help draw entity relationship diagrams, UML diagrams, flowcharts, network diagrams, and many other diagrams. It is also possible to add support for new shapes by writing simple XML files, using a subset of SVG to draw the shape.

It can load and save diagrams to a custom XML format (gzipped by default, to save space), can export diagrams to a number of formats, including EPS, SVG, XFIG, WMF and PNG, and can print diagrams (including ones that span multiple pages).

Xmind | mind-mapping software

"XMind is a brainstorming and mind mapping software tool developed by XMind Ltd. of Hong Kong. The program is intended to assist users in capturing ideas, organizing various charts, and share them with collaboration. It supports mind maps, Ishikawa diagrams (also called fishbone diagrams or cause-and-effect diagrams), tree diagrams, organization charts, and spreadsheets. It can be used for knowledge management, meeting minutes, task management, and GTD." [1]

Besides the Mind Map structure, XMind also offers Org-chart, Tree-chart, Logic-chart, and etc. These charts will play an important role in some circumstances. For example, Org-chart can show the company/department/team structure clearly. Logic-chart is very useful in sorting the reasons. More important, all these charts can be used within one map! Every branch can have the most proper structure, even every topic. [2]

Freemind | mind-mapping software

"FreeMind is a free mind mapping application written in Java. FreeMind is licensed under the GNU General Public License. It provides extensive export capabilities. It runs on Microsoft Windows, Linux and Mac OS X via the Java Runtime Environment.

As with other mind mapping software packages, FreeMind allows the user to edit a hierarchical set of ideas around a central concept. The non-linear approach assists in brainstorming new outlines and projects as ideas are added around the mind map. As a Java application, FreeMind is portable across multiple platforms and retains the same user interface, causing some amount of variation from the common interface on each platform." (Wikipedia)
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Mahara | ePortfolios

Mahara is an open source ePortfolio and social networking web application originally created by the government of New Zealand. It provides users with tools to create and maintain a digital portfolio of their learning, and social networking features to allow users to interact with each other. Mahara content management system provides users with blogs, a resume builder, a file manager and a view creator - a tool to help users create arrangements of their content in a particular way for others to see.

The eportfolio site http://myportfolio.school.nz, provided by the New Zealand Ministry of Education, free-of-charge for all New Zealand schools, is powered by Mahara

VLC | Media Player

This is the Swiss Army Knife of video players. It will play just about anything you can throw at it:

"Simple, fast and powerful media player:

• Plays everything: Files, Discs, Webcams, Devices and Streams. 
• Plays most codecs with no codec packs needed:
• MPEG-2, DivX, H.264, MKV, WebM, WMV, MP3... 
• Runs on all platforms: Windows, Linux, Mac OS X, Unix... 
• Completely Free, 0 spyware, 0 ads and no user tracking. 
• Can do media conversion and streaming."

(Link)

Koha | Open source library management system

"Koha is the first free software library automation package. In use worldwide, its development is steered by a growing community of users collaborating to achieve their technology goals. Koha’s feature set continues to evolve and expand to meet the needs of its user base.

In use worldwide in libraries of all sizes, Koha is a true enterprise-class ILS with comprehensive functionality including basic or advanced options. Koha includes modules for circulation, cataloging, acquisitions, serials, reserves, patron management, branch relationships, and more."

(Link)

Geogebra | open source Mathematics software

What is GeoGebra?
GeoGebra is free and multi-platform dynamic mathematics software for all levels of education that joins geometry, algebra, tables, graphing, statistics and calculus in one easy-to-use package. It has received several educational software awards in Europe and the USA.

Quick Facts

• Graphics, algebra and tables are connected and fully dynamic
• Easy-to-use interface, yet many powerful features
• Authoring tool to create interactive learning materials as web pages
• Available in many languages for our millions of users around the world
• Open source software freely available for non-commercial users
  (Link)

Scribus | Desktop Publishing

Easy-to-use, powerful desktop publishing that allows people to produce professional-quality brochures, magazine layouts, newspapers, advertisements etc. easily from just about any computer.

"Scribus is an Open Source program that brings professional page layout and desktop publishing to Linux, Mac OS X, and Windows with a combination of press-ready output and new approaches to page design.

Underneath a modern and user-friendly interface, Scribus supports professional publishing features, such as color separations, CMYK and spot colors, ICC color management, and versatile PDF creation." (Link)

Jokosha | open source Garageband alternative

Jokosher is a simple and powerful multi-track studio. Jokosher provides a complete application for recording, editing, mixing and exporting audio, and has been specifically designed with usability in mind. The developers behind Jokosher have re-thought audio production at every level, and created something devilishly simple to use. (Link)

Stellarium | Stars, Planets, Constellations

Stellarium allows you to place yourself anywhere in the world to watch the constellations travel across the sky above. Stellarium also allows you to pause, fast-forward or rewind time to see what the stars do. (Link)

There are some very cool videos on youtube that showcase what it can do: