Amazon has introduced file syncing to its Cloud Drive app for OS X, which essentially makes Cloud Drive Amazon's version of Dropbox.
Mac users can install the app on OS X 10.6, OS X 10.7 or OS X 10.8, and drag files in and out of the Cloud Drive folder. Any file put into the Cloud Drive folder will be available on any Mac or Windows machine running Cloud Drive with your login details. The files are also available through any web browser.
If you're already using Dropbox or Google Drive, there's little reason to switch to Amazon's Cloud Drive. However, Cloud Drive users do get 5 GB of storage space for free, which is equal to Google Drive's initial offering but more than double Dropbox's 2 GB of storage for new users.
Cloud Drive for OS X is a free download.