3D2D-IMGUI

Github readme should explain what it does: https://github.com/wyozi/3d2d-imgui

What about 3d2d-vgui?
If you need complicated controls and fine grained control (and the default VGUI skin), you should use https://github.com/HandsomeMatt/3d2d-vgui/
If you need simple UIs that are quick and easy to create and modify, this library should work well.