isn't there a way to render with a transparent sky in Blender?

I think i recall only seeing this done once somewhere but i can’t remember where.
Often times when people make a still image of like a landscape or something i know that they will use flat images of trees with transparency to save themselves some time and still get great results.

I’m trying to make an animation and the area is quite large so i made some trees for the animation purposes, but i think i should put flat tree images in the background off in the distance to save on poly count.

i could take a picture of the tree i made and cut out what’s needed in photoshop, but i’m up for learning things and isn’t there a way to make the sky in blender, or the background of your render be transparent? That way i could just render an image of the tree by itself without having to tediously cut out the unwanted bits?

Thank you (:

Cycles renderer = Enable Transparent in the Render / Film panel
Blender Render = Set Alpha to Transparent in Render / Shading panel

If you render to an image you need to use RGBA and an image format that can use transparency such as .png

Oh, that was quick and easy (:
Thank you very much! ^-^