Ask HN: What to do with Spare PC?

With that spec,I would still sell it and spend the money on something you'll actually use.

Install FreeBSD on it (it's very similar to OS X), configure as file server to back up your Macs, make it your proxy server, firewall, etc to increase your security.

Since you will probably run it headless most of the time, if it has integrated GPU, then you could sell the GeForce card.

you can use it to run a home server, though it would be a waste of it's computing power to use it just as a server

Sell it, use it as a server, repurpose for a NAS, use it to game remotely with Parsec.

Donate it to a school.

You can donate it or rent it out.