Keyhole:

"fuses high-resolution satellite and aerial imagery, elevation data, GPS coordinates, and overlay information about cities and businesses to deliver a streaming, 3D map of the entire globe."

You want to watch the demo movies of this application, and here’s the system’s architecture. Imagine when your optical nerves will be connected to the Internet. That’s another type of client to adapt rendering to. MSN might block you, calling you a daltonian when it’s obvious they can’t tell apart red from green, but I disgress. Of course you’ll need a braintop firewall (desktop firewall for your brain) to protect your eyeballs from frying because of saturated visual virii. Remember the Pong-like stuff from the eighties? Now imagine that stuck into your brain, overlayed on everything you’re looking at. Symantec might have to hire surgeons to handle critical cases.
Anyway, you’ll be able to retrieve a lot of additional useful data and project it on top of your vision. Knowing when to turn it off will be a challenge. And you’ll feel completely at a loss when the system is offline for maintenance (as in, "we’re broken and don’t have a clue when we’ll get the damn thing fixed"). Here’s an exercise for you: think about that today when you walk down the street, and tell me what kind of data you’d like to have in real time about your surroundings. I’ll post the good stuff here. And no, I’m not asking about the obvious stuff such as: "is she wearing a D cup?" (Olivier, that’s an offensive joke to do, bad oh bad). I want walking directions for a start.

11/29/01 update: here are some suggestions by Mike Gold: shopping list match-making and sales notification (when in front of a store), general meta data (e.g. when looking at an item in a museum or a store, or listening to a song), contact list information (if someone you know is in viewing distance, point a text bubble to them, with any relevant notes), information on surroundings (nearest ATM/toilet/police officer, neighborhood crime rate).
12/18/01 update: John Robb mentions small chips that could eventually get pervasive, as Vernor Vinge’s localizers. Read A Deepness in the Sky, you’ll see these tiny devices have hidden powerful features!
12/20/01 update: Write here, write now (on GPS-localized messages "pinned in mid-air").
04/04/02 update: Power Play.
29/11/02 update: for the record, a few links about RDIF.
10/27/04 update: Google buys satellite image firm Keyhole.