I've been a user of FolderShare on my multiple Mac computers for quite a few months now. FolderShare is a FREE application that is provided by Microsoft (they acquired the FolderShare company in November, 2005). It does run on a Mac, but the Mac version is very old (not a Universal Binary, so it has to run in PowerPC emulation mode), and showing its age a bit.
OK, it's really called Windows Liver FolderShare (beta).
FolderShare mostly works, though. And, when it works, it works very well. It's a peer-to-peer way to synchronize your folders across multiple computers. What that means is that Microsoft never owns a copy of your files - they flow directly between your own computers, using 256-bit AES encryption (good enough). They limit each "share" to 10,000 files, which is a limit that I have absolutely run into. And it was not pretty unraveling the mess that was made by blasting through it (likely user error, but it was not entirely obvious how to recover from it).
I am bold enough to use FolderShare to even synchronize the active source code that I am editing across my desktop and laptop. This is a bit scary, but all my stuff is backed up by Subversion and Time Machine, so it's all good.
Dropbox is still in Beta and, unfortunately, I don't have an invite yet. However, the video demonstration looks very slick. It's different from FolderShare in that it does store your files "in the cloud". By storing your files centrally, it also adds the ability to version your files - basically providing a backup service along with a synchronizing service.
It appears that you have to place files within a "Dropbox" folder in order for the product to work. That's different than FolderShare, as FolderShare can work in whatever existing folder you specify. Since Dropbox is in Beta, and I really haven't seen it, who knows how it actually works. Stay tuned for more later (when I get my Dropbox invite, that is).
Also in Beta, and currently only available for Windows, I haven't had a chance to kick the tires on Syncplicity either. But I know the founder well, and have had numerous long talks with him about the product, the product philosophy, the target customer, the market, the competition, and where Syncplicity wants to go.
At this point, Syncplicity feels like the perfect combination of FolderShare and Dropbox. Like FolderShare, Syncplicity allows you to work within any folder - you don't have to specify a side folder to do your backup/synchronization. Like Dropbox, Syncplicity integrates tightly with the Explorer/Finder to work the way you want to work, and also provides a cloud-based, secured version of your files (and the requisite benefits of having that).
I look forward to watching how these products evolve. There is a definite need for something like this, and I do think the need goes way beyond the tech crowd.