Excalidraw is quickly becoming my main tool for illustrating quick concepts and creating wire frames.
it's super fast and lightweight. give it a spin. https://excalidraw.com . Its open source and you can just run it on localhost.

Ecosystem and product builder.

0xa5ad.twitter

Great tool. Here's another one: https://www.tldraw.com/

1. Take all things as seriously as you take yourself.
2. Don't take yourself too seriously.

This is a good illustration of impact of small details on the mental model of a product and hence how users react to it. 
The stroke/font selection on excali gives me permission to be rough and focus on concepts vs how it looks. TlDraw didn’t so it didn’t click for me. It’s a snap judgement.