We have recently discovered this simple (and free) way of syncing our unity project folders on the mac and pc. As a bonus it also backs everything up.
As other people might find it useful I thought I'd share it.
we got a free 2GB backup plan from www.getdropbox.com
This program creates a folder on your computer that gets backed up online.
You can then install it on another computer and it syncs the two folders for you - meaning you can jump back and forth between the two (of course you could do this if you're working on a network as well, but it's a good alternative if you can't network for some reason).
And if you ever accidentally delete anything you can go online and redownload it.
The only issues I've found with it is when I haven't let it finish uploading before shutting the computer down - or downloading before opening unity - this causes unity to complain about missing meta-data files. And sometimes I've tried to delete something only to find that the other computer re-uploads it - but it's only happened a couple of times.