Lexica - An open source word-search game.
yEd - yEd is a free desktop application to quickly create, import, edit, and automatically arrange diagrams. It runs on Windows, Mac OS X, and Unix/Linux.
Crosswords Arena - Word game, multiplayer and computer players
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.
Quackle - Open-source word game similar to Scrabble. Features a board editor, analysis tools, and artificial intelligence that rivals the best players in the world!
draw.io - Online diagramming application