Learn how to create trees for your scene with the Sapling Generator script in this tutorial by Enigma246.
Been requested quite a few times, but wanted to wait till 2.70 was out so the new UI didn't throw anyone, but now that's all sorted it's time for my latest video.
The Sapling addon has been around for ages, but only recently did I actually start using it and realise how easy it is to make great looking trees with it. Even with basic materials(the thumbnail you can see only has single colour diffuse materials with no textures) you can get pretty awesome looking scenes.
About the Sapling Generator:
Modelling realistic trees can be a difficult task, especially if a large number of similar trees with small variations are required. This addon uses the method developed by Jason Weber and Joseph Penn in their paper "Creation and Rendering of Realistic Trees" to easily create a wide variety of trees. Using curves as the basis of construction allows dynamic geometry which can be changed depending on the level of detail required.
The Sapling Generator is bundled in Blender - you can activate it from the preferences.