Siri Assistant is a free app that uses impressively accurate voice recognition to help you complete a wide variety of tasks, from making dinner reservations to calling a taxi to sending yourself an e-mail reminder.
Despite the hard-core DARPA technology behind it, Siri Assistant has a disarmingly simple interface, which looks like a scrolling IM conversation that you're having with the app. The idea is that "Siri" is your assistant, who you speak to directly in the second person (or, less fun, by typing): "Where can I find a good dentist?" will pull up a list of nearby dentists with user-provided star ratings (and reviews, if available), along with buttons for calling each dentist, sharing the info via e-mail, and mapping their locations. "What's the weather in Seattle this weekend?" will pull up a ton of local meteorological info in seconds. Siri shines with even more complex requests, like "I need reservations for a romantic Chinese dinner at 8 on Tuesday for two," which will pull up a list of restaurants (described as "romantic" in reviews) with buttons for available times, which you can then confirm through OpenTable with a couple of taps.
Siri can also autocompose tweets for you (even autosoftening profanities with asterisks), attempt to answer weird questions (like, "How do I take care of a caterpillar," usually by serving up a relevant Web search), and--perhaps its best, semi-hidden feature--Siri can send you an automatic reminder for a future event, either via e-mail or an alert on your phone (e.g., "Remind me about my dad's birthday on March 6"). Siri draws data from and interacts with a whole host of third-party sites, depending on the nature of your question--from Yelp and Yahoo to Taxi Magic, AllMenus, Rotten Tomatoes, Flightstats, and MovieTickets.com (and if you already have an account with a particular service, Siri will remember that after the first time you log in).
Of course, because Siri relies on these services, it's only as good as the information it gets from them. Siri also has some trouble in noisy environments, and finding the best way to ask questions can take some experimentation. That said, its interface and accuracy are much better than similar existing services--and its overall effectiveness gets us one step closer to the AI butler of the future.