iCloud is perhaps the most advanced personal cloud service out there. It comes with 5 GB of free storage. That's more than it sounds like, though: Your Apple-purchased music, apps, books, and TV shows, as well as your Photo Stream, don’t count against your storage quota.
Apple's iCloud gives you more than just storage and an online music server; it also includes all of Apple's wireless services. These include contact synchronisation (derived from the rapidly nearing its end of life Mobile.me), its own email service, mobile backup and location awareness. The last translates into a service that can find your missing iPhones and iPads.
Basic iCloud services are available via the Web on any platform. To really use it to its full potential, though, you need to be running a Mac with Lion, a Windows Vista or 7 PC with iTunes 10.5, or an iPad, iPhone or iPod Touch running iOS 5. Personally, I'm still waiting for Snow Leopard support.
Additional space is priced at $20 per year for 10 GB, $40 per year for 20 GB, and $100 per year for 50 GB. In addition, Apple's new iTunes Match Service lets you access music you own, no matter how, that has a match in Apple's iTunes library.