Microsoft has come a long way lately when it comes to open source. Apple seems to be the only major company that is still not an enthusiastic participant.

Perhaps Apple's contributions aren't as visible, but just off the top of my head here's two major open source contributions from them: Swift[1] and FoundationDB[2]

[1] https://github.com/apple/swift

[2] https://github.com/apple/foundationdb