Some examples of projects to contribute (ordered by languages/main technology,
then thoses chosen in the past) :

(l.15 ) 1. Projects using Java
(l.51 ) 2. Python
(l.109) 3. C
(l.119) 4. Rust
(l.126) 5. C++
(l.148) 6. General list for starting
(l.175) 7. Misc (unordered)
(l.230) 8. Past projects

==============================

1. Java

  Arduino IDE
  https://github.com/arduino/Arduino/

  cucumber-jvm: a pure Java implementation of Cucumber (tool used to test and
  validate specifications)
  https://github.com/cucumber/cucumber-jvm

  elasticsearch: a distributed RESTful search engine built for the cloud
  https://github.com/elastic/elasticsearch

  jabref: bibliography reference manager
  http://www.jabref.org/
  +https://devdocs.jabref.org/teaching

  JGraphT: a library which provides mathematical graph-theory objects and
  algorithms
  https://jgrapht.org/

  JOSM: an extensible editor for OpenStreetMap
  https://josm.openstreetmap.de/

  NextFlow: Data-driven computational pipelines Implemented in Groovy (Smooth Java integration)
  https://www.nextflow.io/
  https://github.com/nextflow-io/nextflow

  Spring framework
  https://github.com/spring-projects/spring-framework

  Tribuo: A Java machine learning library
  https://github.com/oracle/tribuo


=====

2. Python

  Ansible: an IT automation system
  https://github.com/ansible/ansible

  BorgBackup: a deduplicating backup program
  https://github.com/borgbackup/borg

  khard: console carddav client
  https://github.com/scheibler/khard

  khal: CLI calendar program
  https://lostpackets.de/khal/
  https://github.com/pimutils/khal

  matplotlib: a library for creating static, animated, and interactive visualizations in Python
  https://github.com/matplotlib/matplotlib

  nilearn: a library of machine learning for NeuroImaging
  https://github.com/nilearn/nilearn

  numpy: a package for scientific computing with Python
  https://github.com/numpy/numpy

  offlineimap: software that downloadsemail mailbox(es) as local Maildirs. and synchronize both sides via IMAP.
  (main task to contribute: porting it from python 2.7 to python 3.7+)
  https://www.offlineimap.org/
  https://github.com/OfflineIMAP/offlineimap
  https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=937184#10

  panda: a data analysis toolkit
  https://pandas.pydata.org/
  https://github.com/pandas-dev/pandas/

  pythran: a python to c++ converter
  https://github.com/serge-sans-paille/pythran

  pytorch: a library providing tensor computation with GPU acceleration and deep neural networks
  https://github.com/pytorch/pytorch

  scapy: an interactive packet manipulation program and library
  https://scapy.net/
  https://github.com/secdev/scapy/

  Scrapy: a web crawling and web scraping framework
  https://github.com/scrapy/scrapy

  scikit-learn: machine learning toolkit
  https://github.com/scikit-learn/scikit-learn

  Tablib: a format-agnostic tabular dataset library
  https://github.com/jazzband/tablib

  vdirsyncer: calendars and contact synchronizer
  https://github.com/pimutils/vdirsyncer

=====

3. C

  mutt: best lightweight email client in the world (objectively)
  https://github.com/pimutils/vdirsyncer

  Tor: project for privacy and against surveillance, tracking and fingerprinting
  https://www.torproject.org

=====

4. Rust

  duniter-rs / dunitrust: rust client of Duniter (free crytocurrency,
  implementation of relative currency theory)

=====

5. C++

  solarus: ARPG engine (strongly inspired by Zelda on SuperFamicom)
  https://www.solarus-games.org/

  ROS: Robot Operating System
  https://www.ros.org/

  OpenCV: Open Source Computer Vision Library
  https://opencv.org/

  FreeCAD: 3D parametric modeler
  https://www.freecadweb.org/

  Notmuch: Thread-based e-mail indexer, supporting quick search and tagging
  https://notmuchmail.org/

  Praat: 
  https://praat.org and https://github.com/praat/praat

=====

6. General lists and for starting

  https://github.com/firstcontributions/first-contributions
  
  https://www.firsttimersonly.com/
  https://github.com/firstcontributions/first-contributions
  
  Une liste de projets pour débuter :
  https://github.com/MunGell/awesome-for-beginners
  
  https://www.codetriage.com/?language=Java
  
  https://www.eclipse.org/contribute/ 
  Eclipse projects
  https://projects.eclipse.org/
  
  https://www.gnu.org/software/ 
  
  https://apache.org/index.html#projects-list
  https://github.com/apache
  
  https://www-archive.mozilla.org/projects/
  
  https://www.silverpeas.org/intro.html

=====

7. Misc (unordered, non evaluated)

https://github.com/javacc/
JavaCC - a parser generator for building parsers from grammars. It can generate code in Java, C++ and C#. 

https://pmd.github.io/#about
PMD is a source code analyzer.

https://spotbugs.github.io/
SpotBugs is a program which uses static analysis to look for bugs in Java code. It is free software, distributed under the terms of the GNU Lesser General Public License.

http://dia-installer.de/
Dia is free software available under the terms of the GNU GNU General Public License, the GPLv2.

https://www.gimp.org/about/
/!\ probably difficult to contribute

https://inkscape.org/fr/apropos/
Inkscape utilise le standard ouvert du W3C : le SVG (Scalable Vector Graphic) comme format natif, et est un logiciel libre et open source.
/!\ probably difficult to contribute

https://cyberduck.io/
Cyberduck is a libre server and cloud storage browser for Mac and Windows with support for FTP, SFTP, WebDAV, Amazon S3, OpenStack Swift, Backblaze B2, Microsoft Azure & OneDrive, Google Drive and Dropbox. 

http://pencil.evolus.vn/
An open-source GUI prototyping tool that's available for ALL platforms. 

https://sourceforge.net/projects/skim-app/
A PDF Reader and Note-taker for OS X 

https://www.videolan.org/vlc/index.fr.html
VLC est un lecteur multimédia gratuit et libre et un système capable de lire la plupart des fichiers multimédias ainsi que des DVD, des CD Audio, des VCD, et divers protocoles de diffusion.

https://github.com/dspinellis/ckjm
Chidamber and Kemerer Java Metrics 

https://www.bacula.org/what-is-bacula/
Open Source Backup, Enterprise ready, Network Backup Tool for Linux, Unix, Mac, and Windows

https://www.bareos.org/en/
Bareos - Backup Archiving REcovery Open Sourced

https://bitwarden.com/
The easiest and safest way for individuals, teams, and business organizations to store, share, and sync sensitive data.

https://gatling.io/
gatling: Performance Testing For Web Applications

https://github.com/highperformancecoder/minsky
https://www.minsky.sf.net
Minsky: Dynamic flow systems program to simulate economic flows
/!\ small community, it might be difficult to contribute (very few issues, no "beginner"tag, no "howto contribute", etc.)

===== 

8. Past projects

2022-2023
  RetroArch (https://www.retroarch.com/; https://github.com/libretro/RetroArch)
  Khal (https://github.com/pimutils/khal)
  PyTorch (https://pytorch.org/ ; https://github.com/pytorch/pytorch)
  Solarus (https://www.solarus-games.org/)
  Angry IP Scanner (https://github.com/angryip/ipscan ; https://angryip.org/contribute)
  Opsdroid (https://opsdroid.dev/ ; https://github.com/opsdroid)
  Jarvis (https://github.com/sukeesh/Jarvis)
  Zulip (https://zulip.com/ ; https://github.com/zulip)

2021-2022
  Zulip (https://zulip.com/ ; https://github.com/zulip)
  Plotly (https://plotly.com/ ; https://github.com/plotly/)
  Oppia (https://www.oppia.org/ ; https://github.com/oppia/oppia)
  Sympy (https://www.sympy.org ; https://github.com/sympy/sympy)
  newpipe (https://github.com/TeamNewPipe/NewPipe)

2020-2021
  Loop Habit Tracker (https://github.com/iSoron/uhabits)
  Teammates (https://github.com/TEAMMATES/teammates)
  Arduino IDE (https://github.com/arduino/Arduino)
  osu! (https://osu.ppy.sh/home)
  Matplotlib (https://matplotlib.org/)
  nuclear.js (https://github.com/nukeop/nuclear)
  Pygame (https://github.com/pygame/pygame)
  Terasology (https://github.com/MovingBlocks/Terasology)

2019-2020
  RedReader
  JabRef
  Jarvis ×2
  jazzban/tablib
  Wordpress
  NewPipe
  Apps-Android-Commons
  LibreOffice
  FredBoat
  PyChess
  git-cola