Usability 
Dropbox is one of the the best ways I've found to back up and share files across multiple computers and devices and within work groups. It's a popular (4 million users) service that provides cloud storage with an emphasis on file and folder synchronization, sharing and collaborative document and media editing across multiple devices. The basic service, which is free, includes 2GB of storage. Paid options at $9.99/month for 50GB of storage and $19.99/month for 100GB are also available.
Files in selected folders are automatically backed up and synchronized with the cloud by Dropbox's client software, which is available for Windows, Mac OS and Linux computers, Android phones and the iPhone.
In addition to the PC and iPhone apps, Dropbox offers a Webapp in desktop and mobile flavors. The desktop site lets you upload, download and view files.
Dropbox's mobile web site is limited to viewing and downloading the files in your Dropbox cloud folders, there is no uploading. Dropbox Mobile is optimized for smartphones but works on most devices. Horizontal scrolling may be necessary on devices with screens less than 240px wide.Average page size is under 20KB (excluding external JavaScript). The site uses JavaScript, if available, for performance and usability enhancement but also works well on devices without JavaScript support.
