Unetbootin allows for the installation of various linux/bsd distributions to a partition or usb drive, so it's no different from a standard install, only it doesn't need a cd. it can create a dual-boot install, or replace the existing os entirely.
I love it, used it when I was on Mandriva. (Used to be a bit hit and miss) It works as long as I format the USB drive FAT32 & set the boot flag. Also read the distro instructions for specific install requirements.