Ubuntu or Knoppix are easy bootable distros people can use/play with to see if Linux is their cup of tea. Both distros have HDD installs, just in case users want to make a more permanent installation. If they're looking for something that will help them in industry later on down the road, I'd recommend Red Hat/Fedora, since that is what the majority of people tend to use (or have been using/advertising for). Lasty, if you want to play with the latest and greatest builds, you can opt for Gentoo.
It's been a little while, but the bootable distros are still the rage with Ubuntu being the most user-friendly/n00b-proof.
