Almost everyone will be aware about Linux Live CD/DVD, and might have used it too, but those who haven't will find this useful.
A CD or DVD which contain a bootable operating system is known as Live CD/DVD, the unique feature of this is that it can run a fully functional operating system from the CD/DVD without installing the same, it don't require a Hard disk to run itself on a particular computer and it wont do any harm to your operating system and files on your hard disk, instead it will create a RAM disk on your system RAM to temporally run the operating system until you exit the operating system.
Your system will return to its older state when you shutdown the live OS, remove the live CD/DVD and restart the system.
These Live CD/DVD's can come in handy when you want to use your system when your Operating System or Hard Disk might have crashed.
You just have to download the ISO image of any live CD/DVD and burn to run it in your system.
Some Live Linux Flavours
Ubuntu 9.10 latest version, released in October 2009
Slax 6.1.2 latest version, released in August 2009
3). Puppy Linux
Puppy Linux 4.3.1 latest version, released in October 2009
Download Puppy Linux
Knoppix 6.2 latest version, released in November 2009
PCLinuxOS 2009.2 latest version, released in June 2009
Debian 5.0.4 alias Lenny latest version, released in January 2010
MEPIS 8.0.15 latest version, released in January 2010
Fedora 12 latest version, released in November 2009
Mandriva One 2010 latest version, released in November 2009
10). Gentoo Linux
Gentoo 10.1 latest version, released in October 2009