In my opinion, the "killer apps" for Linux (aside from the Apache httpd that amyjess mentioned) are: * A mostly Unix-like OS that runs on available commodity h

In my opinion, the "killer apps" for Linux (aside from the Apache httpd that amyjess mentioned) are: * A mostly Unix-like OS that runs on available commodity hardware, rather than requiring you to buy a SPARC, Alpha or POWER machine, you can buy a commodity x86 machine and use that instead. Because of economies of scale, commodity hardware is always going to be cheaper. * Low (really no) licensing cost. The traditional model of scaling was that when your computer wasn't fast enough, you bought a faster computer. Linux's licensing model makes it affordable to scale horizontally -- to have lots of "slow" computers provide the same computing capacity as one big computer. In a "buy a computer and you have to buy my UNIX" world, any savings you can achieve by scaling out instead of up are offset by software licensing costs. Not in a Linux world, though.