This will be interesting too once they launch: https://www.expressivesuite.com/
(web based motion graphics + a vector tool).
a reply to the top comment: https://news.ycombinator.com/item?id=32850953
TIL of penpot from that comment - seems great tool!
Oh, tried to find it on the page but must have hidden the comment. I've never worked in it, but played around with it for a few minutes and it certainly seems capable enough for solo or small team use. It's also the only open source prototyping tool I've found yet.
[0] http://penpot.app