I'm looking for a few resources on creating level editors, specially helpful libraries or tutorials on the topic. I'm planning to write a small 2D level editor, something similar to Tiled, as a learning exercise for my game. But i also intend to write a level editor for a few 3D experiments, so, it would be nice to see some info. Thanks people! :)

The book "Game Coding Complete: Fourth Edition" has a section that covers setting up a C# level editor for 3D. It shows how to interface with your game engine .dll file (which was made in C++), how you can place objects, etc. I drew from this setup myself and it's worked out well so far!

take a look to this using MFC and OpenGL

http://www.gamedev.net/page/resources/_/technical/apis-and-tools/creating-3d-tools-with-mfc-r1358

Absolutely what Serapth said. Either Qt, if you really want to. C++ is very nice and all that.

Or, as I would personally recommend, C#. It can't get any easier than that. Making an editor is generally not the fun part, so the fact that it's easy to do can be a huge deal.