Dropbox was one of the first cloud services to break through to the mainstream. Earlier this year it boasted 4,000,000 users.

Dropbox works by creating a folder on the user's hard disk, the contents of which are automatically and invisibly synced online whether files are saved to the folder or updated. The benefit of this approach is that other computers can also sync to that account, making it easy to share files among multiple machines regardless of their physical location. Additionally, Dropbox is a pretty effective backup solution and it keeps histories of each file, making it possible to revert to an earlier version.

As the service crosses the line between consumer and small business usage, it's easy to see how for remote workers who can . An intelligent versioning system means that data isn't lost should two or more people simultaneously edit a file.

The new Selective Sync feature is designed for computers with limited storage, such as netbooks or Windows-equipped slates. It works on a per-folder basis, but a notable omission is that it isn't possible to specify individual files. Therefore, users will have to be clever with folder creation if they intend to store large files they don't want to sync everywhere.

Somewhat annoyingly, it's up to each client to specify which folders they'd like to sync to a particular computer, usually done during setup--although Selective Sync can also be accessed later on via the Preferences menu. It isn't possible for a folder creator to specify that it shouldn't be synced on other machines. Ultimately the Dropbox developers have gone for a simple approach, but it reduces the flexibility some users might need.