A notion that a computer can be useless by day and useful by night. Example: slave away at your job programming databases in MS Access under NT by day, and work overtime programming a yet cleaner version of ncurses tetris under Linux by night. The beauty is that, while your computer is one and the same, it almost literaly has two personalities. The idea can be extended for multiple boot configurations: NT, Win9x, RedHat, Slackware, Debian,

configure your pc to boot multiple operating systems in five simple steps:

1. Set up multiple partitions on your hard drive
   use your favorite partitioning utility (usually fdisk, i prefer DriveSetup) and be sure what you're doing because moving partition borders usually renders your data inaccessible. Most tools warn you before they do something like this, though.
2. If you want to run some flavor of Windows, install it to the first partition
   and make sure that one's marked 'active'. Most other operating systems do fine on any partition (i think NT and 2000 do so, too, at least i had 98 and NT running dual-booting some time and can't remember too much trouble) (addition: actually, windows seems to accept any partition as long as it's the first active partition found. grub can modify the "active"/boot flag at boot time, so you could even run multiple instances of win95 from one disk (for whatever reason you'd want to do that...))
3. Install the other operating system(s).
   My favourite is BeOS and 'of course' i've got some kind of Linux, but you're free to choose whatever you like (that is, whatever will run on your PC). Multiple installations of the same OS will usually do fine. Two flavours of Windows are also common.
4. Install a boot manager
   like grub, lilo, or bootman. note that installing Windows usually overwrites the master boot record and you will have to install it again. It can be very useful to have a boot disk or bootable cd-rom from which you can install this boot manager, especially when you have to re-install windoze
5. Enjoy choice!