What are my options for syncing a folder between a Mac and a PC?

Solution 1:

Dropbox does have an option called LAN sync, which syncs files between computers on a LAN directly. However this still also syncs them to the cloud, it just saves you some internet bandwidth.

As @woz suggested, a file share on either computer or a NAS (including Time Capsule) would do the job. You could also run rsync from a cronjob, or use GoodSync, which has a neat GUI for that.

Solution 2:

The closest thing I have found is Cubby. It's a Dropbox competitor, but it has a feature known as DirectSync that allows you to sync one or more folders across computers (Mac or PC) without uploading them to Cubby's cloud.

The downside? From Cubby's FAQ: "Cubby always sets up direct UDP connections between clients unless your firewall or NAT configuration prevents it from doing so, in which case data is relayed through our data centers using a TCP based, E2E secure SSL tunnel." IOW, it might run through their servers, which means it's not a perfect solution for my situation. Still, it's close.