Let's assume that you're a person in 2011 with a smart phone and a Facebook account (or Twitter, or a blog etc.). You may not know it yet, but you're creating a diary. Walking around, taking pictures of your kids, updating your status, "checking in" to the park or restaurants - all of these things create a digital time-trail of where you've been and what you did. Momento for the iPhone is an ingenious little app that takes all of that stuff and aggregates it into a diary - automatically.
I've never kept a diary, and although a few years ago I tried tweeting all the stuff I did each day (I was inspired by the Twitter account of John Quincy Adams). It lasted all of a day or two. But I do create Facebook status updates fairly regularly, as well as post pictures, check in to Foursquare and blog. Momento takes all of those things and then puts them into a calendar so I can see what I was up to.
Once you give it your information, it will go back and grab as much as each service will provide, in many cases going back and getting months or years worth of stuff. It's really fun to go back and see what you were complaining thinking about a few months ago, or what your kids were up to, or what you thought was important at the time. You can also enter diary entries ("new moments") directly into Momento for those moments you want to keep but don't necessarily want to share with the rest of the world.
Though it will import from nine different services plus RSS/Atom feeds, there are a few other services I'd like to see. I'd love it to aggregate content from my Instagram feed for instance - or go back in time and find my TextAmerica Moblog images. (Is it even possible for anyone get those back?) It's also not optimized for iPad, which is unfortunate because I think it would be really cool optimized for an iPad - especially for viewing photos. An even better feature would be the ability to purchase a printed diary of what you did for an entire year (or other specific time frame). A skeuomorphic "virtual" diary transformed back into a real paper diary would be awesome.