Since around 2012, I have had an ongoing interest in learning about game engines. Particularly open source ones. For the purpose of settling on one and using it for a major project. I've even tried my hand at making my own game engines. In the earlier days, I was unhappy with a lot of game engines, hence my desire to make my own. But recently, a lot of game engines have really started to shine and I'm quite spoilt for choice now.
Godot has really come quite far in the last 2-3 years. It is now a very serious contender to Unity, especially for indie game developers. It's my current go-to for most projects, however I find fun in using other game engines as well.
A very old game engine, but it's still quite useful today. Good for basing your own game engine off of it.
The only (current) game engine that takes Kotlin files and compiles it into native, web (via JS) or JVM. This makes it a very versatile engine.
This was the main open source competitor to Unity until Godot. It's still good for mobile game development.
Recent discovery. I like it because it's similar to code I've been working on for 2 years :O
I backed the 32Blit handheld game console, and I have one on the way. I'm looking forward to making game(s) for it. In particular, porting my current game, Wizards with Rockets.
Other game engines that are quite interesting, but don't quite make my shortlist of engines of choice
Unity - Nice but bloated, proprietary, requires separate editor for coding, Editor stuck in the early 2010's (for now)
Love2D - Good for smaller games, but I don't really like making full games with Love2D/lua as you basically need lots of third party libraries (eg. Classes or ECS), and it becomes very cumbersome
SDL2 - Ideal from a certain perspective, but more complex API, less features than Allegro 5
jMonkeyEngine - Good for making 3D Android games, but not really interested in that atm
Urho3D - Technically impressive, but a bit complex to set up
Unreal Engine 4 - Seems best suited for 3D games, not my thing atm
Defold - Very geared towards mobile games, I want to make desktop games currently
Orx - Requires thinking in a different way to make games (data-driven), but very versatile and has many good features (inc live-reload)
Amethyst - Still very new, need to learn Rust
Leadwerks - Not as interested in making stand-alone Hammer editor style games. But I own it.
Construct 3 - Great for rapid prototyping with potential to deploy to multiple platforms, but need to pay ($13+ per month) to get the most out of it.
PySDL2 - Still SDL2, but a bit faster to code. But with less deployment targets.
Clanlib - Objectively good, but not as much fun as other game engines to develop with
There may be more, but those are the ones that stand out to me off the top of my head.