The Internet Drive
There has been some discussion about google/amazon/apple/microsoft/and-everyone-else providing storage on the Internet that you can use to store files and move them around between places. Personally, I think thattheres a much more interesting solution here.
The idea is that every person dedicates a small portion of their disk to a "cache", say 50% of their free space (which would dynamically change of course). When they access a file off the "Internet Drive", they locate it off a computer somewhere that has it, and download it and store it in their cache, so that further accesses to the same file happen at the speed of the local disk. When you modify the file it is replicated to x other computers elsewhere, thus if any one computer fails (or is just "offline") then it can be restored from another computer. x can be selected on a per file basis (maybe 1 for temporary files, 3 for "normal" files, 10 for important files).
Each person gets their own personal "Internet Drive", so when you install the software you have a blank drive where you can store files. However you can make a "seed" file of another file or directory. These seed files can be sent to someone else via email, or posted on a website or whatever. The recipient "plants" the "seed" file in their drive and they get access to that file or directory. Want to share those photos with your family? Put them in a directory and send them a seed of the directory. Want to access your Internet Drive from work? Just send yourself a seed at work that you can plant.
Except in special circumstances, people have a small working set of files that they use, only a few hundred mb. Of those they modify an even smaller set. A lot of the larger files are usually shared amongst multiple computers, and are never modified. Files that are modified are usually only modified by one person, and that person can't be in two places at once.
If your computer is disconnected from the Internet then you can still continue to work as normal, when you reconnect to the Internet the computer will deal with expiring stale files and backing up modified ones. If a conflict occurs, then you punt to the user, let them know that there is a conflict and let them deal with it, it should be a rare enough occurance that noone should worry. Coda showed that this was quite possible.