Of the four projects I listed, only Swarm and IPFS overlap in functionality.
Ethereum is a decentralized, consensus-driven data store and execution environment.
Whisper is a decentralized messaging protocol, which you'd use for data that doesn't require consensus. Consensus is expensive and relatively slow, but communication between parties doesn't require it.
Swarm and IPFS are content-addressable, decentralized file transfer systems. You'd use one of these to store the HTML, CSS and JavaScript that implement an application on top of the other decentralized systems. Or just to store arbitrary static files.
It's hard to shut down a system that lives on thousands of computers and can be accessed by typing the name of the site into a web browser. That's the experience that's motivating people to build these systems, and they're going to change the way the world works.
