Windows Live Mesh
Microsoft offers another free cloud storage service, called Windows Live Mesh, supporting both Windows and Mac OS X. Similar to Dropbox, it's designed for synchronizing files among multiple computers. Additionally it offers synchronization of your Internet Explorer and Microsoft Office settings, as well as Remote Desktop capability so you can use your computers anywhere on the Internet.
The Windows Live Mesh service offers a tremendous amount of storage space, theoretically 10,000GB. But the catch is that you can only have up to 200 folders. Each folder can be up to 50GB in size and contain up to 100,000 files. These limits may or not may be a problem for you, depending upon what you do on your computers and how you organize your files. In Windows, you can quickly check the number of subfolders and files contained in a folder by right-clicking the folder and selecting Properties.
To set up the Windows Live Mesh service, you install the program on each computer you want to sync. Then you can add folders to be synced and select the computer(s) with which you want to sync that particular folder. The folder(s) will then appear in the personal user directory of the computer(s) you choose.
Windows Live Mesh itself lacks sharing features, but you can use 5GB of SkyDrive space with the Windows Live Mesh service, which is in addition to the 25GB of regular SkyDrive storage. This feature is useful if you want traditional web-based access to your files, and sharing features similar to what SkyDrive provides.