Desmos is the web-based interactive geometry program that isn’t GeoGebra. It’s very popular with teachers.
Someone’s made a nifty tool to turn a Desmos construction into an animated gif. It’s called – you guessed it – GIFsmos. They’ve got a blog containing a few nice animations, but it doesn’t seem to have been updated since I discovered it in March. Anyway, the tool still exists, so go and see what you can create!