I do remember a "smart NIC" making its rounds in the news a few years ago -- although "few" in this case might be >10. It was essentially a low-power machine in itself (with no Raspberry Pis in sight yet) that could finish an HTTP download or receive e-mails without waking up the host computer. But that's more high-level than the buffering/relaying of packets you are thinking of.

Edit: a bit weird, I wrote a very similar comment a year ago about this item. Now I feel slightly compelled to find the news source for it...