It's Windows Live Sync, a free service from Microsoft that syncs folders across various computers. It works with Macs and PCs and is completely free, though it does require a Windows Live ID.
Setup is pretty easy and is all web-based, meaning changes to your sync setup can be made from anywhere. The service uses peer-to-peer networking, so there's no limit to the size of files and folders you can sync.
The downside to that is that the computers need to be on for sync to work, since it uses P2P. But if you have computers that stay on (or are at least on most of the time), then it's an incredibly helpful little tool. I use it to sync my photo editing project folders. So I have a "Photoshop stuff" folder on all three computers that I use, but each one had different projects in them. Now, Live Sync keeps the folders all up to date, so I can start a project one one machine and finish on the other.
It even does folder merging. So there's no need to rename folders or create new ones. In my example, I started with three individual folders, each with separate content, but Live Sync simply merged the folders together, adding whatever files each folder lacked so they now all contain the same files. Live Sync runs in the background, syncing whenever a file changes, so all you have to do is hit save and your files are synced.
Live Sync works with both Mac and Windows machines. I tested both versions and they work flawlessly, syncing to and from each other without issue. The P2P transfering is really fast, but the fact that computers need to be online to sync with each other is a slight bummer, but it does mean that file size, online storage, etc. are all non-issues, which is nice.
Lastly, Live Sync has one more hugely useful feature: remote access. If enabled, you can browse the files of any of your connected computers from anywhere using just a browser. So if you're at work and need a file at home, you're covered. If your home computer has Live Sync installed and remove access enabled, you can simply log-in to the Live Sync site, browse to the file you need, and download it. Really nice.
So if you have folders that need to stay synced, I highly recommend checking it out. It works very well and has already made my life easier. :-)