What's The Greatest Software Ever Written?

Summary: Unix (and especially BSD 4.3)

I hate articles which ask a question in the headline and then answer it only in the last few paragraphs.

Duh! I wouldn't be reading my Lions book if it weren't.

"Hello World!"

The first compiler. Thank God for that thing. If it hadn't existed...

... it would have eventually.

And then that would have been the first compiler. Thank God it existed.

Emacs, because it's still around after 27 years and in everyday use.

Edit: had the number of years wrong, was originally based on the original emacs and not GNU emacs.

Didn't even have to read the article to guess that the answer is Unix