Better late then never they say... so today I bring you the news that the grand-daddy of point and click adventure game creation software, AGS, has been open-sourced recently.
For AGS there are literally thousands of fan made games available (picture taken from this one), and even though most of them (all?) are not FOSS, it is definitely nice to have the engine and the editor now under the pretty liberal and GPL compatible Artistic License 2.0.