Box.net has expanded its online document collaboration service with the ability to synchronize files on a Windows desktop with those in the cloud.

The Box Sync feature, launched Thursday, is targeted at businesses where employees keep a large number of documents stored on their office computers. The new service makes it easier for those employees to work on documents together, without leaving their cubicles.

To use Box Sync, a customer must first download the Box Sync client, which creates a "My Box Files" folder under "My Documents" in Windows Explorer. To sync folders stored in the cloud on Box.net with the desktop, a user only has to right click the Box folder and select "Sync Folder to Desktop."

An image of the chosen folder appears in the "My Box Files" folder, and any changes made to files in the Box folder will lead to a notification to a person's collaborators. Changes to desktop files are automatically transferred to the same files stored on Box.net.

In addition, folders dragged and dropped from the office computer to the "My Box Files" folder is automatically uploaded to Box.net, with the same synchronization features in place.

In cases where two people edit a document at the same time, Box Sync can merge simple changes and save copies of a file, so collaborators can review potential version conflicts later.

Nordic River launched in February launched a TextFlow application to make revision management easier Box.net's cloud content management system. Integrated with the Box.net system, TextFlow makes it simple to see changes side by side and choose which one to accept.

Box.net plans to roll out Box Sync in waves to its business and enterprise customers at no charge throughout the summer. The company also plans to make a modified version of Box Sync available later in the year to users of the site's free version and individuals who pay $9.95 a month for 10 GB of Web storage and a file size limit of 1 GB.

Business collaboration services are available for $15 a month per user for 45 GB of storage and a file size limit of 2 GB.