I'm using a 3D open-source game engine called BDX. It's a 3D Java-based engine built with LibGDX that interfaces and works with Blender as the editor. It's integrated really well, so there's no "Import / Export" process manually involved - BDX handles everything.
It's pretty fun and easy to use, and has a lot of cool stuff that I'd miss if I jumped elsewhere, like gamepad support, input maps (with gamepad support!), 3D physics, a component system, animated sprites, draw batching, 2D filters, filter downsampling, 3D shaders, and, obviously, integration with Blender. Even despite those nice additional features, the engine itself is rather small, as it's built on top of existing frameworks and programs (LibGDX, jBullet, I think, etc). This size makes it easier to understand the functions and features of the engine, and also makes it easier to work on it or add new features.
I've contributed quite a few commits to the engine to improve and patch it up, and it's improved a lot since I first started using it around a year ago. Actually, one of the games I published here on itch, Kyro, was made in BDX, which is cool.
Anyway, check it out!
EDIT: Oh, and Aseprite is a cool cross-platform open-source sprite editor; nice animation tools, in particular.