I'd like to install Ubuntu (Desktop or netbook edition, preferably latest version), onto a laptop using a small USB stick. (480 MB free space.)
How can I do this?
There is a dedicated article on this:
You can install a command line (CLI) Ubuntu using the Alternate Install or Mini iso (which should fit on your 512mb stick)
Then install Crunchbang using the alternate method: http://crunchbanglinux.org/forums/topic/26/crunchbang-linux-81001-alternative-installation/
You could also try the Lite version of CrunchBang, which is less than 512mb.
You could use Unetbootin to prepare the USB stick.