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