If you want something readily available, mature, and polished for Windows, the news isn't good right now. I've been curious about this type of software as well and have been lazily keeping an eye on a few projects. I'll tell you what I know, but I haven't used all of these. Most are still under active development and not all are stable.
New Entry: 2012-Dec-15 SeaFile is a new cloud sync service with all the standard trimmings: 5 GB of free space, syncing between clients, versioning, sharing, encryption (optional), and also includes collaboration. The server and client are both open source, allowing you to host your own server. The server software is Linux only. Clients are currently available for Windows, Linux, and Mac OS.
If you don't need iOS support (yet), there's SparkleShare. If you do and you've got the requisite skill set, I'm sure they'd appreciate the help. :-) It's using Git as a storage backend. This lets you use your own Git server or any public or private Git repository. I assume that the end goal is for servers to be available on Linux, Mac and Windows and clients on the previous 3 along with Android & iOS.
Update: SparkleShare recently released a Windows version. I haven't had a chance to test yet to see if it's client-only, client & server, or requires Git to be installed as well. All I've found is that there's an
.msi available to download.
New Entry: Cubby is a relatively new service from LogMeIn that lets you synchronize folders across the internet with or without the cloud. It's currently in invite-only beta, but it seems that the wait for an invite isn't all that long. People are trading invitations as well. Any folder can be monitored by the program. Clients available for Windows, Mac, iOS, and Android. One of its greatest strengths is that it will let you do peer-to-peer sync of folders across the internet without being forced to use their cloud service. The gain is that you're not reliant on the cloud, the downside is that you lose some of the "cloud" features (sharing, versioning, backup). That feature is now called DirectSync and apparently behind a paywall.
IQBox is another project along the same lines as SparkleShare, but uses Subversion as a storage backend rather than Git. Currently in Alpha on Windows, no other clients available.
Acid Rain is another of the same, only using Mercurial for the backend. Server is Linux only and a Windows client is available.
AeroFS is another "Dropbox on your own systems" setup. It's pretty early in development as well, and currently invite only.
Syncany is a cloud syncing application that allows for a wide range of backend storage (Local Folder, FTP, IMAP, Google Storage, Amazon S3, Rackspace Cloud, WebDAV, Windows Shares, SFTP). Still in heavy development, primary development target is Linux but Windows appears to be a high priority as well.
My gut says that SparkleShare and Syncany have the potential to become the primary projects that the OSS communities support (and $deity knows that there can't be only one -- it wouldn't be proper OSS without a holy war :-) ), but that's purely a feeling with no evidence to support it.