PlantUML for drawing all kinds of diagrams based on formal text descriptions (online version: https://www.plantuml.com/plantuml/uml/, but mainly just an offline tool). I’d encourage people not to use online services for everything but simply rely on good tools.
Freesound.org (https//freesound.org) for sound fragments
This reminds me also of Framasoft.org , who do a similar thing.