As in the title.

Unreal Editor 2.0 was what UT99 used and it's literally my only experience making maps.

I adored the CSG for how intuitive it was but sadly Unreal Editor 2.0 is not open source.

I know its a very dated technique but I don't mind that at all.

I could not find anything similar to it.

Everything is either too heavy, tied to a specific game or doesn't have CSG.

Any similar editors would be appreciated too.

Even a 3D CSG library in C or C++ would be appreciated, since the only one I found was in Python and had completely inactive GitHub repo and bugs.

Have you tried the Unreal 4 engine?

I said I don't want anything too heavy or tied to a specific game. I don't want to jump into huge engines, I want to experiment and have fun, not push out a game in record time.

I am not aware of anything engine agnostic. In my opinion Unity is probably your best option and then use some of their CSG plugins. Just search for CSG in their app store. Another option might be GTKRadiant. This is tight to old Quake games, but relatively light weight and open source and you might be able to just export the brushes (CSG geometry). If you look for Quake editors you find some simple stuff as well.

http://icculus.org/gtkradiant/

http://kristianduske.com/trenchbroom/index.php

I said I don't want anything too heavy or tied to a specific game. I don't want to jump into huge engines, I want to experiment and have fun, not push out a game in record time.

I meant using it as a editor only, but you are right it's a bit heavy in size for only using as a level editor.

Maybe use Blender, although this is the one modeling style that Blender is known for being bad at, it can do the basics and you can create your own brushes; it's also light weight. Max would be the best 3D software for this but it's a heavier option than Unreal.

Any particular reason for the -1, so I can know where I was wrong? I like learning new things, it's why I browse this site. :)

I've found this but it's not out yet: http://runegear.wildrune.com/#/home

This is more along the lines of what I'm looking for.

Another thing I could use and that still beats anything mentioned so far is copy pasting from Unreal Editor 2.0 because it saves a plaintext format to clipboard that I could then parse. :P

But that's also forcing tools into spots they weren't mean into.

For 2D there is Tiled but for 3D there seems to be nothing, even though most 3D level editors are very similar and seem to have a lot overlap between each other, especially with brushes.

Hello, I'm the creator of Runegear.

We have the same mindset about level creation and that's why I am building this editor!

If you have any requests please state them so I can take them into consideration.