I had previously seen Apple's visionary 1987 concept video of the Knowledge Navigator (6 minutes). That was twenty-three years ago, but it's remarkably modern in various ways, and there are a fair number of things in it that we still can't do today.
But now there's a free iPhone app that does a bunch of pieces of it: Siri Assistant.
Here's a video demo (3 minutes). It kind of looks like magic. I downloaded it and tried it out, and it kind of feels like magic, too.
At the Semantic Technologies conference in June 2009, Siri's CTO Tom Gruber gave a keynote presentation (45 minute video) describing the motivation and goals and implementation of the app (plus comparisons to the Knowledge Navigator), and it's pretty amazing. He talks, among other things, about how performing certain very common tasks (like making dinner plans) normally requires visits to six or eight different websites; Siri takes a spoken natural-language query and does all those website visits for you, using the websites' APIs, and gives you useful results.
Yes, yes, I know that speech interfaces are nothing new. Most of what you can do with Siri, you can also do on other major smartphones by phrasing your searches carefully and jumping around from one application or website to another. But Siri (a) provides a nice natural-language (limited-domain) interface; (b) integrates all those websites (Gruber says it's like "a mashup with gravy"); and (c) is aware of info about you (like your location), info about context (like the current time of day where you are), and info about the conversation so far (like whether your new query is an elaboration on your last one). All of which makes it feel really smart and really convenient.
[Example added later: say it's 11 p.m. and you need dinner. You can say "Late-night dinner" to Siri, and it'll respond, "Ok, here are some restaurants which reviews say are open late within walking distance."]
The app is referred to as a "Virtual Personal Assistant," but I'm inclined to call it a "Digital Concierge."
Turns out Apple just bought Siri, the company that makes the app. I'm hoping this means that (a) the app will become even more awesome over time, and (b) the technology will get integrated into the iPhone, which already has some interested speech-recognition features but could use some improvement in that area.
(Thanks to macrumors.com for info and links.)