SugarSync is a for-pay service that extends your local files seamlessly to your other computers, to a secure webpage, and pretty much to anywhere you think they ought to go. CEO Gibu Thomas describes SugarSync as "get my stuff everywhere," a supercharged and cross-platform version of .Mac iDisk synchronization. Our colleagues at Download Squad have checked out the service in detail, and there's a good writeup at webware.com as well.
To use SugarSync, you can define a set of folders to sync, photos to share, "Magic Briefcase" files that live in the cloud and sync down on demand, click and go -- then the defined content will be available on all your computers. With both "full sync" and "light sync" modes, you can leave rarely used files on the SugarSync side and only retrieve them as needed (great for those MacBook Air users hungry for storage). You can also selectively share photo galleries or other content via the web UI.
Once you get past the 45-day, 10 GB of storage trial period, you can use the discount available through 4/15 to get a half- off data plan (starts at $25/yr for 10 GB, up to 250 GB for $250/yr).The advantages of SugarSync for backup, specificity of synced items, and a supported client may make you give it a look as an alternative to iDisk, or to cloud solutions like OmniDrive, JungleDisk or other clients for Amazon's S3 that require you to roll your own synchronization plan.