godot

i recommend this, because it has the python programming language
and it's not an intricating engine.

But if you're coming from a unity background, don't worry, it has the c# language
(with some slight changes to the library)

Great Engine!

It's free, Open source, easy to learn and very powerful!
Try it out!

Awesome game engine for 2d and 
in the next update it seems it will super super powerfull 
for 3d with the release of 4.0

It is also very good for begginers and for fans of python as it uses an amazing python like programming language called gdscript

Great Engine!

It's Free, Open source, easy to learn, very well optimized for old Computers
and very Powerful!
I like it you should try it!