Mac: The downloads folder on a Mac can get pretty messy. MacWorld shows off an automator script that automatically keeps that folder organised and clean.
Chances are that the bulk of your downloads fall into some obvious categories: videos, images, disk images and archives. This script automatically organises downloads of those types into whatever folder you choose. You can also do this with software such as Hazel, but the Automator script is free and built right into OS X. Head over to MacWorld for the full guide.