Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
Today, open-source software is everywhere but many peple still think of it as being relatively new. It's not. Open-source software actually goes back decades.
So, with no further adieu, from the most recent to the oldest, here's my list of older significant open-source projects.
1) Linux: August 25, 1991
There were thousands of open-source programs before Linus Torvalds wrote "I'm doing a (free) operating system (just a hobby, won't be big and professional like gnu) for 386(486) AT clones." to the Minix Usenet group and unknowingly started the operating system juggernaut that became Linux. That said, Linux has also become the most successful of open-source programs.