Well CUPS did not originate with Apple, they bought it when they needed a mature printing system. It existed for BSD prior to Apple's purchase.
I don't know if the BSD's have made any use of anything from Apple's Darwin release? Perhaps it's not possible due to the licencing (Apple Public Source Licence) which is copyleft'ish.
There is quite a bit of sharing between Apple and FreeBSD. For example libdispatch has been ported to FreeBSD. Also when Apple imported pf into Darwin; FreeBSD used that same port to update their port of pf from OpenBSD.
As others have mentioned, stuff like the TrustedBSD and Audit framework came from Darwin, and features such as Seatbelt and sandboxes are slowly being considered or improved upon by Capsicum for example.
Well CUPS did not originate with Apple, they bought it when they needed a mature printing system. It existed for BSD prior to Apple's purchase.
I don't know if the BSD's have made any use of anything from Apple's Darwin release? Perhaps it's not possible due to the licencing (Apple Public Source Licence) which is copyleft'ish.