One of the best online syncs I have ever come across is Dropbox. Extremely useful for work and play. When working with someone not on the same LAN dropbox can easily sync (and keep in sync) a folder between you and a friend. In fact, I have 6 users on dropbox that I have folders synced with. Anytime my wife needs a mp3 and I’m at work, I just drop it in the dropbox and a few seconds later its synced to hers.
How it works: Makes a folder on your computer (PC or Mac) and syncs it to their server. Anytime you want to sync a folder with a friend, it creates a new folder (ex: Chris-Corey). When you take your laptop off-line it seamlessly starts syncing where it left off once the internet connection is re-established, even mid file. Very helpful if you constantly transfer files from your work computer to home computer. It’s completely free with 2 gigs of storage. The more people you refer, the more space you get. So, go get Dropbox.