ZendTo is completely free of charge, requiring no licence, installation or subscription fees. Free assistance is provided through mailing lists and email.
ZendTo was originally based on the "Dropbox" package written by the University of Delaware, but has since changed almost beyond recognition. I am eternally grateful to them. It does not have, and never has had, any relation whatsoever to the commercial service available at www.dropbox.com. That is a totally different product, and is nothing more than an accidental name clash a very long time ago (in a galaxy far away).