This GPIO Pinout is designed to be both a quick and interactive reference to the Raspberry Pi GPIO pins, plus a comprehensive guide to your Raspberry Pi's GPIO interfaces. It also includes dozens of pinouts for Raspberry Pi add-on boards, HATs and pHATs.

A largely browser based diagram and chart webapp written in Javascript. Use Ant to compile it, then zip it up and upload it to a web server. You can even use Python's BaseHTTPServer to play with it. Forces HTTPS. I was able to build it on my laptop pretty easily. Gives you the option to save to a number of different storage back-ends if you install the back-end webapp, including your local machine.

There is also an Electron webshit version for download.

Online version, free to use: https://app.diagrams.net/

A chart and diagram webapp written in HTML5 that lets you do pretty much whatever Lucidchart does. Export to PNG, GIF, and JPEG.

A website that lets you draw ASCII diagrams with your mouse.

An encyclopedia of data visualization methods, charts, and paradigms. explanations When to use what chart for what kind of data and why.