What are my options for syncing a folder between a Mac and a PC? Suppose I have 2 computers attached to the wireless network in my house, one a Mac and one a PC.  Suppose I want to continuously sync a folder on the Mac with a folder on the PC.  What options do I have?

My first thought would be to use something like Dropbox, but in this case I don't want the files to leave my network and go to the cloud.

Edit: NAS not available here, nor is an external drive.  Just the 2 computers.
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.