Zapr is a combination of a FTP and web server. It allows you to share and synchronize your files with other computers or with other people. It's free of charge but appears to work on on Windows XP and newer computers. Unlike other systems that provide the storage, you're using your own computer. That of course means it needs to be left on.
In addition to file sharing, the system to transfer files too large to be emailed. You send a link provided by Zapr in your email and upon clicking, the file is transferred from your computer's hard drive.
If you are have computers that cannot network with each other, this would be a viable way to share files. They say their system is secure. You may want to read the fine print. Your system could be vulnerable if there is any weakness in their software.