https://draw.io - Basically web-based Viso for drawing diagrams. Does not require signing up for an account - you can bypass everything and just start drawing. I hadn't used it in years, and today I was on a customer call and brought it up. I couldn't remember how to do free-hand drawing, but everything else was obvious enough that I was able to do what I needed to do, without reading the doc.
https://godbolt.org/ - compiler explorer. Compiles snippits of code using a wide variety of compilers and shows you the resulting assembly code. Great for exploring optimization levels.
https://www.mcternan.me.uk/mscgen/ - Generator for Message Sequence Charts (MSCs), which are a way of representing entities and interactions over some time period. Integrates with Doxygen.
https://miro.com - collaboration whiteboard tool. I played just a bit with flow chart (non-collaboratively) and I like the drawing UI. Lowest cost paid version is a bit expensive ($8 per month per team member), but I like that they offer (undisclosed) discounts for non-profits.