Altair BASIC, in its first incarnation, MITS 4K BASIC, was a true milestone in software history—the first programming language for the world's first truly personal computer, the MITS Altair 8800. It was also the very first product, the foundation stone in fact, of Microsoft (then Micro-Soft) – the world's largest software company at the beginning of the 21st century.
Written by Bill Gates, Paul Allen, and Monte Davidoff, without access to an actual Altair computer or even an 8080 CPU (they used a self-made 8080 simulator running on a PDP-10), it fit nicely into 4 KB of memory leaving enough room - several hundred bytes - for BASIC programs. The historic interpreter was later expanded to MITS 8K BASIC, and eventually, Altair Disk Extended BASIC (for use with MITS' floppy disk drive).
Altair BASIC was also the source of 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 within the Homebrew Computer Club. When Gates and Allen finished Altair BASIC, they wanted to sell it. But just before it shipped, John Draper managed to get hold of a paper tape containing the program, 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 them 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.