Amazon's Cloud Drive was unveiled two years ago for the primary purpose of letting customers store music files purchased from Amazon. Although you could upload other files to Cloud Drive, it wasn't a replacement for cloud storage services like Dropbox because it didn't automatically sync files from one computer to another.

Today, that limitation is gone. "The new app for Windows and Mac now includes File Sync, which lets users easily store and access files in the Amazon Cloud from a folder on their computer," an Amazon spokesperson told Ars in an e-mail. "Files in this folder are automatically stored in Cloud Drive and can be accessed from a computer or the Web. When you install the app on a second computer all of your synced files are available there as well. The updated app makes it simple for users to put files in Cloud Drive and to access them from any of their computers, ensuring that they always have access to the latest version of their files from home and on the road."

Like Dropbox, Google Drive, or Microsoft's SkyDrive, the Cloud Drive desktop application (download it here) creates a new folder in your file manager. The desktop application supports all Windows versions from XP to 8 and Mac OS X 10.6, 10.7, and 10.8.

The application requires Java. If you attempt to run the application on a Mac without Java installed, you will be prompted to install a Java runtime. On Windows, the application apparently comes with an embedded Java runtime. Java security flaws are well-known, but are mainly a problem with the browser plug-ins, which Cloud Drive doesn't require.

File syncing is desktop-only for now. There is a Cloud Drive photo storage app for Android, but it hasn't been updated since February 1. Amazon told us it has "nothing specific to share" about plans to bring file syncing to iOS, Android, or other mobile platforms. Dropbox, Google Drive, and SkyDrive already support smartphones and tablets.

Cloud Drive offers 5GB of free space with the option to buy more. Dropbox provides 2GB free, while Google Drive provides 5GB and SkyDrive provides 7GB.

In Amazon Cloud Drive, 20GB costs $10 a year, 50GB costs $25 a year, with several more tiers all the way up to 1,000GB for $500 a year. All the prices work out to $1 per year for every 2GB. The Amazon prices are identical to SkyDrive's, slightly cheaper than Google's, and about half the price of Dropbox.