XMPP. In case you haven't noticed, I'm a big fan.
XMPP is so versatile that if it becomes widely adopted it will be to the Internet what HTTP was: a platform for new types of applications. And where HTTP as a platform is a server-centric model, XMPP is capable of peer-to-peer communication.
Remember what happened when everyone got HTTP clients (they're called browsers

) ? The Internet exploded. Well, if everyone gets a full-fledged XMPP client I think you can expect roughly the same thing to happen.
One of the most fascinating features of XMPP is the way things are addressed. EVERYTHING is addressable over the network. You can talk directly to ANYTHING, and ANYONE. I can't stress how big of a shift that would be from the current model. It's HUGE.
Let's take a step back and think about this for a second.
i. Right now I cannot send text directly to your instant message account (unless you're using an XMPP-based client), I have to send the message to your IM server which relays the message to you.
ii. I cannot send audio directly to your phone, the phone company has to route it there.
iii. I cannot share a picture directly with your Facebook account, I have to sent it to Facebook first to be carried on to you.
iv. I can't send a file directly to you, I have to put it on a share or email it to you.
(Not to mention the fact that these are all disconnected, you can't combine these into a single message stream. XMPP addresses that problem very nicely, as the wave client shows.)
XMPP removes these intermediaries from the network. Social networks and proprietary transports no longer have an exclusive license to deliver content, the clients talk directly to one another.Do you see the difference ? There are no longer social networks or any other type of networks required to relay the communication, we are now down to exactly 3 components :-
i. Clients
ii. Storage
iii. Applications
Of course there is always the underlying dumb pipes that transport the data, but from a functional perspective the network has been normalized out of importance.
Clients can be whatever we need them to be. It can be the Google wave client, it can be your phone, it can be a desktop app. These will evolve over time, but the Google client is a fantastic starting point, certainly light years ahead of anything else that's available today.
Storage becomes a utility, something you pay for as you go. I already use this model myself for backups, I shoot them up to the Amazon cloud and pay for the amount I use. As time goes on my communication--audio, video, pictures, text--will be stored there as well, and I'll use it in my waves as needed. (Note that waves do NOT embed this content, they link to it and the client downloads and renders it in place.)
And the applications. This is really exciting, because just about every application in existence will be transformed by this quantum shift in the network topology. Applications now interact with your client and provide input to your communication stream, and output to your storage. They will become a facet of your communication, not a completely disconnected activity. You will communicate with apps much in the same way that you communicate with people, and they will communicate with you.
Take CRM and ERP systems for example. Instead of customers emailing you about a sale and then sending purchase orders, it will be part of the "sale wave". The entire sale, from start to finish, will be encapsulated in a single wave, bringing individuals in and out of the conversation as need. The ERP and CRM platforms themselves will be participants in this conversation, recognizing the purchase order, executing the workflow, processing the order, making the order details available to manufacturing or delivery in a sub-wave, and then making the receipt available to the customer and the sales team. Your CRM Whether you approve the purchase order from your desktop, your phone, or a point of sale device, makes no difference--they can all be directly addressed and participate in the conversation natively.
That's just one example off the top of my head, but I truly believe that every software application in existence will eventually need to be re-architected to be much less application-centric and much more communication-centric.