OS X uses the FreeBSD userland, but it was more or less a one time pull, the kernel is radically different, so it would be hard to keep things synced; as a result, I doubt there has been a lot of contributions of patches backwards. However, Apple and FreeBSD both had license problems with gcc, and Apple sponsored LLVM, which benefits FreeBSD as well. Maybe there's a cultural benefit for FreeBSD as a result of OS X exposing people to the FreeBSD userland, although running into limitations that have been fixed upstream more than ten years ago may give people the wrong ideas.