StereoKit is a really nice and comprehensive open-source XR toolkit, which can be an alternative to Unity/Unreal for smaller projects, prototypes or maybe even teaching and hackathons. It leverages OpenXR to support different XR devices.
It’s a code-only framework (C#, or C when using the underlying library directly) that can be used to quickly set up an application that loads a few assets, sets up some interactions and renders a scene. Hand interaction, and things like loading 3D files, come out-of-the-box. Code can be deployed fairly easily to Quest, HoloLens 2, Magic Leap 2, or run directly on a PC with an OpenXR headset (such as a Varjo XR-3 or 4) attached.
Usable from Windows and Linux (Mac possible, but the simulator doesn’t work). StereoKit is also under active development.
Not recommended when you need any kind of advanced lighting (there’s not even shadow mapping) or complex scenes with lots of interactions. Think of it as a light version of the usual game engine, in library form.
I’ve been using it at SURF for demo work for years. Being able to quickly re-compile, deploy and test is really nice. The immediate mode UI is also quite a breeze to use.
Coincidentally, Maarten Struijk Wilbrink is doing a StereoKit workshop in the XR ERA community, 13 March 2024.