Ubuntu One is an online backup, storage, streaming and file sharing service.
Despite the name, you don't have to use Ubuntu (or know anything about it) to use the service. It offers 5GB of free online storage space to anyone who installs one of the clients, and there are builds available for Ubuntu, Windows, Android 2.1 and higher and iPhone/ iPad iOS 3.1 or higher.
Once installed the program works much like many other cloud storage clients.
There's a local Ubuntu One folder where you can save files and have them automatically synced with your online account, and you can also synchronise other folders if necessary.
A "Share folder" option in your browser-based dashboard will send a link to the email address you specify, allowing the recipient to see and download any of those files.
Handy settings include the option allow you to limit Ubuntu One's upload and download speeds, so it won't hog all your network bandwidth.
And there are commercial add-ons available, if you need them. Music streaming gets you 20GB of storage for $3.99 a month, and streams content to Ubuntu One's Android or iPhone apps. Or you can add extra storage in 20GB blocks for $2.99 a month, or an annual $29.99.
An easy-to-use cloud storage client, particularly interesting if you need to sync files across Windows, Ubuntu, Android and/or iOS devices.