I recently came across Excalidraw Whiteboard in a course I am taking, where it was used to create a variety of architectural diagrams. The outputs have a pleasing, hand-drawn feel and the onboarding is as simple as can be: just load up the site and you can immediately start using the full product, no account or setup required.
The style reminds me a little bit of XKCD, which further reminds me of the R package xkcd, intended to produce XKCD-like plots with ggplot2.