When open source software pioneer Linus Torvalds first posted the Linux source code on an internet newsgroup site in 1991, he was clear about its terms of use: no one should be able to make money from Linux, he said.

So how has the operating system come to be big business for some of the world's leading IT vendors? After all, almost two decades on, the altruistic Linux ethos remains largely intact.

The software is still open source, in that every line of code is openly available. It is still free, so any organisation can download and use the operating system without charge. And the Linux kernel itself remains proudly independent of any commercial interest, in that it is not "owned" by any one company, but developed on a voluntary basis by a worldwide community of contributors.