Altair BASIC was a true milestone in software history : the first programming language for the world's first truly personal computer. If that alone wasn't distinction enough, it was also the very first product, the foundation stone in fact, of Microsoft - the world's largest software company.
Altair BASIC was also the source of a controversy in the late 1970s. At that time, most computer owners traded programs with each other with no thought to buying programs. This had been the norm in Homebrew Computer Club. When Gates and Allen finished Altair BASIC, they wanted to sell it. John Draper managed to get hold of a paper tape with Altair BASIC on, just before it was launched, and went on to make lots of copies and give them away for free. He went so far as to state that he'd give it to anyone, as long as that person would make two new copies and give away.
This made Bill Gates furious and he went on and wrote an open letter to the computer community denouncing piracy. He had two problems: that Draper had stolen a tape and copied it, and that the tape that he had gotten hold of was an early buggy version, making Altair BASIC look bad.
- Bill Gates' open letter: http://www.tranquileye.com/cyber/1976/gates_open_letter_to_hobbyists.html