• geometry batching - ​http://www.nvidia.com/docs/IO/8228/BatchBatchBatch.pdf
• planetary rings - ​http://johnwhigham.blogspot.com/2011/11/planetary-rings.html
• icosphere - ​http://pastebin.com/aFdWi5eQ
• compressed textures - ​http://www.opengl-tutorial.org/beginners-tutorials/tutorial-5-a-textured-cube/

GUI

• Turbo badger - ​https://github.com/fruxo/turbobadger

Minecraftlike

• ​http://dev.minetest.net/Main_Page
• ​http://www.volumesoffun.com/polyvox-about/
• ​http://www.sea-of-memes.com/LetsCode93/LetsCode93.html

Light and Shadow

• Irradiance Environment Maps (Olgierd's suggestion) ​http://www1.cs.columbia.edu/~ravir/papers/envmap/
• indirect roguelike map shader - ​http://pastebin.com/pTGsx4vf

PBRT

• PBRT book - ​http://www.pbrt.org/
• ​https://github.com/mmp/pbrt-v3/ -- PBRT book code v3
• ​https://github.com/mmp/pbrt-v2/ -- PBRT book code v2

• pbr intro ​http://www.codinglabs.net/article_physically_based_rendering.aspx
• pbr-cook-torrance ​http://www.codinglabs.net/article_physically_based_rendering_cook_torrance.aspx