iCloud does this and, in my experience, it's been pretty good about it. I have yet to run into a file that I needed that wasn't already downloaded locally. Sinc

iCloud does this and, in my experience, it's been pretty good about it. I have yet to run into a file that I needed that wasn't already downloaded locally. Since it uses files based on timestamps, it does it reliably. The only thing it's missing is the ability to lock a file so that it's always available, regardless of last access time/date.