Educational

Blinken (C++, GPL-2.0, mature)
Childsplay (Python, GPL-3.0, mature)
Garith (C++, LGPL-2.1, mature, inactive since 2013)
GCompris (C, Python, GPL-3.0, mature, inactive since 2016)
ORIENT (C#, Java, GPL-2.0, mature, inactive since 2009)
Prescription Wars (ColdFusion, JavaScript, GPL-3.0, mature, inactive since 2016)
PrimeShooter (JavaScript, GPL-2.0, mature, inactive since 2010)
Q (Python, GPL-3.0, mature, inactive since 2013)
thiefcatcher (C++, GPL-3.0, beta)
Tux of Math Command (C, GPL-3.0, mature, inactive since 2011)