I'm pretty sure Matlab will last forever. There is a healthy company backing it with technology lock in from many, many companies where programming is not their core competence.
Does the browser stack (HTML/CSS/JavaScript) count as software? If so, I don't see any of those going anywhere. The world wide web probably isn't going to be replaced anytime soon, and web browser fundamentals probably aren't going to change either.<p>Then again, programming languages and scripting languages in general seem like they'll last forever. Sites and programs may come and go, but the languages they're written in will probably be around for the long haul.
The Linux kernel. Just because it’s everywhere right now. Even in 1000 years when hardware has moved on, someone will want to virtualize/emulate some hardware that was running Linux.
I don't think much software will be useful even for one century because of competing implementations, changing requirements etc, but Github could be an archaeological treasure-trove one day.
Nothing lasts forever. No software, no programming languages, no operating systems. They will all pass out of use.<p>I see only git, vim, and emacs lasting until the end of my life.