The three or four most obviously influential pre-Linux open source projects are:

- MIT's X Window System
- CMU's Mach (and Andrew filesystem)
- BSD Unix
- The FSF's GNU project (gcc, gdb, emacs, etc.)

There's the whole digression about BSD vs. AT&T that's wildly not interesting to your question, but broadly those are the three big things that really had a huge impact. Dig deeper and there are plenty of smaller but still quite influential systems (e.g., the Glasgow Haskell Compiler got its start in 1989), but what's certainly striking is the seemingly central role of academia in jumpstarting open source.