These effects are interesting for games or presentations:

Lens flare effects: http://www.doughogan.com/wordpress/wp-content/uploads/2009/05/flare_examples.png

Gravitational lenses: http://en.wikipedia.org/wiki/File:Black_hole_lensing_web.gif

Fisheye lens: http://en.wikipedia.org/wiki/File:Vlg_shop.jpg

http://farm1.static.flickr.com/164/346456520_ac3578c4fc.jpg