GrassTechnique

Grass in-game can be created using a variety of techniques.

Grass texture from a high-poly model.
Image by Eric Chadwick.

Tutorials

Tutorial: Modeling low-poly grass for Cryengine 3 CryDevForums thread. Tutorial for creating grass in CryEngine.
Ryse Moss Tutorial Polycount forum thread. Tutorial about how the moss was created for Ryse.
Grass in DOTA 2 Polycount forum thread. Discussion about how the grass was created for Dota 2.
Hand Painted Grass Tutorial by Mark "Jenn0_Bing" Foreman. How to bake high-res grass geometry into tiled textures.
Realistic Mowed Grass - ideas? Polycount forum thread. Realtime grass techniques for both long and short grass.
Rendering Grass in Real Time with Dynamic Lighting from Kévin Boulanger. Research whitepaper on rendering grass efficiently for realtime use.