A method that approximates the vegetation, in this case the trees, with billboards when viewed from far away. This seems to work like a more advanced impostor as it uses more than one billboard per tree/object. The image below shows to the right how the tree is split up into multiple billboards.

Link where you could find the paper:
http://graphics.uni-konstanz.de/publikationen/2005/hybrid_level_of_detail_approaches/Colditz%20et%20al.%20–%20Real-Time%20Rendering%20of%20Complex%20Photorealistic%20Landscapes%20Using%20Hybrid%20Level-of-Detail%20Approaches.pdf

This algorithm for rendering grass allows user interaction with the grass which most other methods won’t. They call this interaction effect treading and it’s implemented as a scalar field which determines in which direction the individual grass straws should bend.

Link to the paper, more screenshots and movies
http://www-evasion.inrialpes.fr/Publications/2003/GPRFC03/

A full article that presents every aspect of implementing billboarded grass fields in games. It uses a vertex shaders for animating the grass in the wind.

Here’s the full article for free (also the source for the image). Or you could buy the great book “GPU Gems 1″ which also contains the article.
http://http.developer.nvidia.com/GPUGems/gpugems_ch07.html

An implementation of the grass rendering by Nvidia:
http://developer.nvidia.com/object/nature_scene.html

 A very advanced method to do grass rendering with dynamic lightning. There are shadows cast from each grass straw and you can paint a density map to select where you want grass and how much. The grass is obviously also animated in a very realistic way. The following is how they describe the technique briefly.

“We use a combination of geometry and lit volume slices, composed of Bidirectional Texture Functions (BTFs). BTFs, generated using a fast pre-computation step, provide an accurate, per pixel lighting of the grass.”

Link to more images and the paper:
http://www.kevinboulanger.net/grass.html