[YayamiOmate]

Appart from siblings' mentions, I think they also opensourced networking libs/protocols, bonjour and cups. Cups should be mentioned especially, since that was a basis for network printing under linux. (I hope im right on this)

While Im not a fan of the corporation, its strategy and most of the policies, some dilligence is due here. They are known to release low level stuff.

[tssva]

CUPS was not originally written or open sourced by Apple. CUPS was released in the late 2000s as open source and soon after became the default print system for most Linux distros. Apple hired the original creator and purchased the source code in 2007.