I've used FolderShare from Microsoft for PC to PC sync for some months recently and pushed it to the limits synchronizing files over Internet larger than 1GB (2GB is the limit) and with a file count of just less than 10,000 files (10k is the limit and a bit low in my opinion) and it has worked really good except a few hiccups. Now there is a new version and a new blog and it should be even better. It has crashed a couple of times though.
Keep files in sync across your computers.
Share folders with friends.
Access your files from any computer.
When I moved a bunch of files from one folder to another FolderShare only sent a move message so the files were not sent over Internet something that would slow down things.
Some features missing are being able to sync the desktop, shortcuts (.LNK), excluding file types, files or folders. It can also be useful if it was possible to synchronize a whole drive.
For sync on a PC only or on a network SyncToy is a better option.
Highly recommended as one of several Microsoft's useful freeware.
Ps. answers to some common questions on the FolderShare blog.