The idea here isn't so much to recommend a specific service (though we'll do that) as much as to give you an idea of what to look for when you're investigating them yourself.
It's the apps
All but one of the services we looked at offer desktop clients for both Mac OS X and Windows; Box.net is currently beta testing its Mac client. In some cases, those client apps give you access to almost every feature the service offers; others let you do little more than control local preferences (such as bandwidth throttling for file transfers). SugarSync's software provides the most functionality, despite its scattered approach; Dropbox provides comparatively little.
One thing to remember about a service's reliance on a desktop client: Unless your collaborators (co-workers, clients, contractors, and so on) already use the service, you'll have to convince them to download and install the necessary software. If they don't, and they use the service's Web interface alone, they may be second-class collaborators.
The services vary widely in the quality and availability of mobile clients for the iPhone and such. Syncplicity and Live Mesh 2011, for example, offer no native smartphone clients; you have to use their mobile Web apps instead. Box.net, Dropbox, and SugarSync, by contrast, provide software for the most widely used smartphone platforms in the U.S.
All of the services except for Microsoft's Live Mesh allow you to manage files via Web apps as well as from desktop or mobile clients; some of them require you to use the Web for some operations (such as restoring old versions of files). Dropbox's Website is very Web 1.0; for example, most actions require a new page to load. Other services offer true Web apps or rely on Flash for quicker, more desktop-like access.
Syncing and sharing
These services vary in what they allow you to share, and how they let you share it. All except Dropbox allow you to share any folder you choose; some also allow you to share individual files. Dropbox opts for simplicity, allowing you to share just a single folder named Dropbox (including that folder's nested contents); there are workarounds to that, including manually creating symbolic links or using a third-party utility such as MacDropAny ( Macworld rated 3.5 out of 5 mice ).
Of the five services we looked at, only Syncplicity limits the number of computers that can sync to a given set of files or folders: if you have a free or personal account, you can sync two computers at a time. Box.net restricts desktop syncing to its paid business accounts. Windows Live Mesh 2011 is unique in that it will let you specify which computers a folder will sync with. Dropbox and Windows Live Mesh 2011 are the only services that synchronize directly between two machines on the same local network, without a round-trip out to the Internet. That's a distinct advantage when you're sharing large files with people in the same office.
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