Windows Live Sync

Windows Live Sync, previously known as FolderShare, is a free service by Microsoft available for Mac and Windows, that allows you to synchronize files and access them from Windows or Mac.

Choose files and folders, assign tags, choose friends to share files with,... Windows Live Sync is a good service and it's totally free.

Don't worry abouut privacy and security, the service is really well designed and uses AES and SSL algorithms to protect it.

Finally, we have to say that it offers up to 4GB and a limit of 20,000 files so we think it's enough.