Artha is a handy thesaurus based on WordNet with distinct features like global hotkey look-up, passive desktop notifications, regular expression based search, etc.. Artha may be used as a free open-source replacement to the proprietary WordWeb Pro.
Rnote aims to be a simple but functional note taking application for freehand drawing or annotating pictures or documents. It eventually should be able to import / export various media file formats.
With Audio Sharing you can share your current computer audio playback in the form of an RTSP stream. This stream can then be played back by other devices, for example using VLC.
By sharing the audio as a network stream, you can also use common devices that are not intended to be used as audio sinks (eg. smartphones) to receive it. For example, there are audio accessories that are not compatible with desktop computers (e.g. because the computer does not have a Bluetooth module installed). With the help of this app, the computer audio can be played back on a smartphone, which is then connected to the Bluetooth accessory.
Marble is a Virtual Globe and World Atlas that you can use to learn more about Earth: You can pan and zoom around and you can look up places and roads. A mouse click on a place label will provide the respective Wikipedia article.
Open source RGB lighting control that doesn't depend on manufacturer software.
Omnilauncher for Linux to help you quickly find and run apps and do other useful things.
MyTourbook is free software to visualize and analyze tours which are recorded by a GPS device, bike- or exercise computer and ergometer.
GoldenCheetah is a software package that:
Downloads ride data directly from the CycleOps PowerTap and the SRM PowerControl V. Support for SRM PowerControl VI and VII is planned for the future.
Imports ride data downloaded with other programs, including TrainingPeaks WKO+ and the manufacturers' software for the Ergomo, Garmin, Polar, PowerTap, and SRM devices.
Provides a rich set of analysis tools, including a critical power graph, BikeScore calculation, histogram analysis, a best interval finder, and a pedal force versus pedal velocity chart, to name just a few.
Is available for Linux, Mac OS X, and Windows. (The Windows version does not yet support direct downloads from the SRM PowerControl.)
Is released under an Open Source license.
We believe that cyclists should be able to download their power data to the computer of their choice, analyze it in whatever way they see fit, and share their methods of analysis with others.
SMPlayer intends to be a complete front-end for MPlayer, from basic features like playing videos, DVDs, and VCDs to more advanced features like support for MPlayer filters and more. SMPlayer remembers the settings of all files you play. So you start to watch a movie but you have to leave... dont worry, when you open that movie again it will resume at the same point you left it, and with the same settings: audio track, subtitles, volume...
Plays almost all formats without the need to download and install any codecs yourself due to it being based on 
MPlayer. Now SMPlayer supports 
MPV as well.
Currently supported hash functions include CRC32, ADLER32, GOST, MD2, MD4, MD5, MD6, SHA1, SHA2, SHA3, RIPEMD, HAVAL, TIGER, WHIRLPOOL and BLAKE2.
X-tile is an application that allows you to select a number of windows and tile them in different ways. It works on any X desktop like Gnome, KDE, XFCE, LXDE. The main features are: many tiling geometries, undo tiling, invert tiling order, optional system tray docking and menu, filter to avoid listing some windows, filter to check some windows by default, command line interface.
Features:
tile checked windows vertically
tile checked windows horizontally
tile checked windows grid
tile checked windows custom
tile checked windows triangle up, down, left, right
tile checked windows quad
undo tiling
invert tiling order
cycle tiling order
maximize checked windows
unmaximize checked windows
close checked windows
filter to avoid listing some windows
filter to check some windows by default
optional system tray docking and menu
command line interface
Strawberry is a music player and music collection organizer written in C++ using the Qt framework.
QDirStat is a graphical disk usage utility. The successor of 
KDirStat without any KDE dependencies.
KeePass Cross-Platform Community Edition - A community maintained fork of the popular KeePassX application with more features and frequent updates.
No excuses. No distractions. Just write. ghostwriter is a feature-rich text editor using Markdown syntax which makes it suitable for note-taking, document redaction, simple HTML website creation and much more.
GPS log file viewer and analyzer with support for GPX, TCX, KML, FIT, IGC and NMEA files.
Gaphor is a WYSIWYG (What You See Is What You Get) UML and SysML modeling environment easy to use that will help you create your applications.
You are able to create nice UML diagrams while it assists you with design decisions. Supports models composed by multiple diagrams and allows to share objects between the. And diagrams are styleable with a CSS-like language!
Dia is a GTK+ based diagram creation program for Linux, Unix, Mac OS X and Windows released under the GPL license.
Dia is roughly inspired by the commercial Windows program 
Microsoft Office 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.
KRename is a very powerful batch file renamer which can rename a list of files based on a set of expressions. It can copy/move the files to another directory or simply rename the input files. KRename supports many conversion operations.
The qBittorrent project aims to provide a Free Software alternative to 
µTorrent. Additionally, qBittorrent runs and provides the same features on all major platforms (Linux, Mac OS X, Windows, OS/2, FreeBSD).
qBittorrent is based on the Qt toolkit and libtorrent-rasterbar library.
Features:
• Polished µTorrent-like User Interface
• Well-integrated and extensible Search Engine
Simultaneous search in most famous BitTorrent search sites
Per-category-specific search requests (e.g. Books, Music, Movies)
• All Bittorrent extensions
DHT, Peer Exchange, Full encryption, Magnet/BitComet URIs, ...
• Remote control through a Web user interface
Nearly identical to the regular UI, all in Ajax
• Advanced control over trackers, peers and torrents
Torrents queueing and prioritizing
Torrent content selection and prioritizing
• Natively integrated: When Downloads Completes Options (no need for additional script or plug-in)
Exit, Suspend, Hibernate, Shutdown
• UPnP / NAT-PMP port forwarding support
• Available in ~41 languages (Unicode support)
• Torrent creation tool
• Advanced RSS support with download filters (inc. regex)
• Bandwidth scheduler
• IP Filtering (eMule and PeerGuardian compatible)
• IPv6 compliant
• Sequential downloading (aka "Download in order")
• Available on most platforms: Linux, Mac OS X, Windows, OS/2, FreeBSD
Free community support by volunteers:
• https://forum.qbittorrent.org
• https://wiki.qbittorrent.org
• https://github.com/qbittorrent/qBittorrent/issues
Zotero is a free, easy-to-use tool to help you collect, organize, cite, and share research.
Extensible, multi-platform mind mapping creation, analysis and presentation software.
A "hierarchical spreadsheet" that is a great replacement for spreadsheets, mind mappers, outliners, PIMs, text editors and small databases.
Zettlr is a supercharged markdown editor that combines many writing editor features and collect them in one application. It aims at speeding up your workflow and help you write without any interruptions. Therefore it integrates all features that one might need during a writing process.
Features:
Great list. It helped me discover a lot of productive tools, which I needed from long ago :)
