Windows Live Mesh stands apart from those other four, because it's free. But it also lacks the kinds of storage and user upgrades you get from those others.
Note that, if you're using these services with people outside your company, you'll need to convince them to pay up--or you'll have to pick up the tab.
The bottom line
For serious-minded business users who want central control and granular access to files, Box.net's business accounts are the clear winner among the services we looked at. Too bad we have to wait for its Mac client to be released.
From the standpoint of simplicity and accounting, I'd have to go with Dropbox. Having a single folder to sync and straightforward options makes it an easy way to collaborate--as long as you don't need fine-toothed file- or folder-permissions. Its LAN Sync makes it great if you're collaborators are all on the same network. Because it doesn't count old versions or deleted files against your storage quota, it's particularly good for storing large, frequently revised project files. The service has a ways to go if it's going to compete in the business market; it needs to get more flexible account terms and some kind of trial membership for business users.
Google Docs's simultaneous editing and superb Web app interface are great--as long as you don't need to sync files on your Mac or other device. For some companies, using a separate service (such as Syncplicity) to enable Google Docs desktop syncing, while using the Web interface as the primary way to edit and collaborate, may be the right combination of cost, features, and reach.
Whichever service you choose, the point is to reduce the amount of time all project members spend managing files, and redirect that effort toward productive work. Collaborative cloud storage should keep project members down to earth, while all the magic happens way above them.
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