Inkscape is an Open Source vector graphics editor, with capabilities similar to Illustrator, CorelDraw, or Xara X, using the W3C standard Scalable Vector Graphics (SVG) file format.

Note: This software is now renamed as "diagrams.net", however, it is still popularly known as "draw.io".

Paint.net is a free and very capable image editing software for Windows. Great alternative to Photoshop for people that do not need all the stuff PS offers.

Web-based vector graphics editor with desktop apps, focusing on real-time collaboration and version control for UI/UX design.

Versatile design tool integrating photo, video editing, and AI-powered features for creating graphics, logos, and social media content efficiently.

Pencil is a standalone app that lets you create diagrams or UI mockups. It doesn't have collaboration/multi-user features.

Draw lets you produce anything from a quick sketch to a complex plan, and gives you the means to communicate with graphics and diagrams. With a maximum page size of 300cm by 300cm, Draw is a an excellent package for producing technical drawings, general posters, and much else.

Excalidraw is an open-source whiteboard tool that lets you easily sketch diagrams that have a hand-drawn feel to them. Available directly online as a web app and also through CodeSandbox to easily see how the code works, and the app is available for self-hosting.

Offline image editor offering brightness, contrast adjustments, vignette, border effects, and more; exports as JPEG or PNG with metadata options.

Privacy-focused app for sketching and digital art. Customize canvas colors and brush sizes, export in PNG, JPG, and SVG—all offline.

Low-fidelity wireframing tool emulating sketching, built for online teamwork, offering drag-n-drop simplicity, reusable components, interactive prototypes, and exportable wireframes.

Square Sketch recreates drawing and writing on graph paper. Instead of a pen, a mouse and keyboard are used, creating precise vector graphics that can be edited flexibly.

Create vector illustrations quickly and consistently with this AI tool. Simply input a prompt and select a style to generate four unique illustrations in under 30 seconds.

SMath Studio is a math program with 'paper'-like interface and numerous computing features. It has an ability to work with systems, matrices, vectors, complex numbers, infinities and fractions. SMath Studio includes integrated mathematical reference book.

AutoDraw is an intuitive drawing tool for everyone. It's a Google Experiment project that provides relevant suggestions as you draw, based on what it guesses you're drawing.