JungleDisk is quite possibly the first serious, easy to set up alternative I've seen to Apple's .Mac iDisk. Using the power of WebDAV, JungleDisk allows you to mount an online, syncing hard drive (with various size options available) on Mac OS X, Windows and Linux, and save and access data to and from it as if you were working with files in a local folder on your desktop. This is all run through Amazon's S3 online storage web service, which is "based on the idea that quality Internet-based storage should be taken for granted". As an extra kick-in-the-pants to .Mac's iDisk, data sent to and from your JungleDisk is encrypted. A software package is available for each OS to make installation and setup a snap.
Pricing is spectacularly low, though I'm slightly confused as to how billing works. They truly mean: "pay only for what you use" - there isn't a flat monthly or yearly fee for your storage and bandwidth. Instead, on their pricing page, they state that you are charged $0.15 USD per gigabyte of storage per month, and $0.20 USD per gigabyte of data transfer. They offer a convenient pricing chart on their main page to compare with other services.
I'm already a .Mac customer right now, as I like the integration of all their other services. But JungleDrive sounds like a great alternative for those who only want the ease of use of an iDisk. If you have already used this service or will try it out soon (bonus points for more than one platform), please feel free to comment on your experience.