I've tried a boatload of different synchronization tools and finally found one that actually works as advertised and meets my needs: Dropbox. Since I'm a very picky consumer (read: perfectionist), it is rare that I say a product is very good.
So what makes this product unique? It works simply, works quickly, and it is free for normal consumption. Considering that my definition of normal is anything but the norm, that's a big statement.
Basically, you install the Dropbox client on your machine (Windows, Mac, Linux, what-have-you) and set the location for your Dropbox files. Anything that shows up there is automatically updated on the server. By using multiple clients with the same account you get real-time-enough (that's a technical term) synchronization across multiple machines.
I use Dropbox for all my OneNote notebooks. And my work documents. And some source code. And some pictures. Pretty much anything I want to have A) backed-up automatically and B) available anywhere.
Your free Dropbox account is limited to 2GB of data, and even for a storage hog like me this is plenty. And did I mention that the synchronization is fast? That's because it looks inside and synchronizes parts of files.
I use a notebook, a netbook, a Mac, and a desktop PC throughout every day for work. As I was typing meeting notes into OneNote on the netbook I noticed it synchronizing in the background. Every few minutes the alert on my desktop popped up that updates were downloaded. Talk about peace of mind. When I'm done for the day, I walk out with my netbook knowing all my machines have the same information on them. I can edit any file on any of the machines and they all just stay in sync about as fast as I can switch from one to the other. When I type a new article on the plane, as soon as I get home, my laptop syncs. In the span it takes for my Mac to boot up, my files are already being downloaded. I can get right to work knowing I'm using the latest versions of the files, hassle-free and worry-free.
For a while I was using the Gladinet software but it wasn't reliable, was too slow, and expensive. The GoodSync tools are really slow. In addition, it won't run on my desktop PC because I run Windows Server (boo!). With both of these, as with most of the sync solutions I've tried, the synchronization algorithm doesn't look inside the files so I was able to confuse the client with multiple updates and then lose some data. Inevitably this happened and pissed me off. And of course both of these cost money too.
Among other things, I keep a Box.net account, a SkyDrive, a private FTP server, and heaps of both Amazon and Google Storage. Most of this is for work and collaboration, and I use them for encrypted back-ups too. But none of these make moving my data around easier than Dropbox. If Box.net (my previous favorite), or any of the big boys wants to know how to do synchronization right, check these guys out.
And no, I'm not paid for this, I'm not affiliated in any way with Dropbox et al. I just rarely get the chance to toot the horn of something truly great and with such mass appeal.