There are three simple techniques to create a nice sky in a game.

• Skybox: Good when the sky can be viewed from all directions. For example in a space game. It is basically a box with inverted faces.
• Skydome: Probably the best choice when rendering outdoor scenes with terrain. For example a driving game. It is basically a half sphere (dome) with inverted faces.
• Skyplane: Sometimes an option when rendering outdoor scenes and you should not be able to see the horizon. It is just a plane with the normal pointed down and a sky texture attached to it.

Information and tutorials about skyboxes:
http://www.toymaker.info/Games/html/skybox.html
http://www.gamasutra.com/features/19981023/bell_01.htm

Information and tutorials about skydomes:
http://www.spheregames.com/index.php?p=templates/pages/tutorials
http://www.riemers.net/eng/Tutorials/XNA/Csharp/Series4/Skydome.php
http://www.flipcode.com/archives/Sky_Domes.shtml

This is a special technique to do impostors. Instead of only storing the color of the object they also store the depth and the normals. The abstract:

“We propose to render geometry using an image based representation. Geometric information is encoded by a texture with depth and rendered by rasterizing the bounding box geometry. For each resulting fragment, a shader computes the intersection of the corresponding ray with the geometry using pre-computed information to accelerate the computation. Great care is taken to be artifact free even when zoomed in or at grazing angles. We integrate our algorithm with reverse perspective projection to represent a larger class of shapes. The extra texture requirement is small and the rendering cost is output sensitive so our representation can be used to model many parts of a 3D scene. “

Link to paper and more screen shots:
http://artis.inrialpes.fr/Publications/2006/BD06/

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

A very nice procedural method to generate buildings for cities that undoubtly could be used in games. The abstract for the paper:

“CGA shape, a novel shape grammar for the procedural modeling of CG architecture, produces building shells with high visual quality and geometric detail. It produces extensive architectural models for computer games and movies, at low cost. Context sensitive shape rules allow the user to specify interactions between the entities of the hierarchical shape descriptions. Selected examples demonstrate solutions to previously unsolved modeling problems, especially to consistent mass modeling with volumetric shapes of arbitrary orientation. CGA shape is shown to efficiently generate massive urban models with unprecedented level of detail, with the virtual rebuilding of the archaeological site of Pompeii as a case in point. “

Link to the paper, videos and a presentation of the technique:
http://www.vision.ee.ethz.ch/~pmueller/wiki/CityEngine/PaperBuildings