I've been using Momento, developed by d3i, for a few weeks now: it's being touted as a "journal-keeping" or "diary-keeping" app, but to me my journal is my Moleskine in which I write volumes of drivel. Therefore, in my world, this app is an innovative cross between that and a sort of private Twitter account or microblog (despite appearances to the contrary, SOME things don't NEED to be twittered!). The first cool thing about it is that you can enter your Twitter, Facebook, and Flickr credentials and it pulls in your activity from those services. Twitter doesn't go back very far, but I had a good time reading and seeing what I was doing a long time ago from the Facebook and Flickr data (it also does Last.fm, but I don't use that so I can't comment on it).
The cooler thing is that you can make new "moments," so called because this is NOT a full-fledged diary, meant to contain pages and pages of your deepest thoughts and feelings. Tap the compose button, and you get a screen with a few lines and a few buttons:
Jot down your "moment," and then you can rate it, tag it with the people and places mentioned, and add your own custom keywords (all of which are accessible through menus later, so you can read all your "Work" entries at once, for example). Snap a photo to add, or add one or more from your library. Result:
In this example, your personal place-tagged moment with photos, then a Twitter entry and a Flickr photo.
The major Achilles' heel with this app is that there's no good way to export the data. It's fully backed up with iTunes sync, but long-term that means little. The developers, however, assured me via e-mail that they're on this issue. Currently you can export your moments as text via a totally unusable .plist file, so I am not counting that.
I hesitated to buy this app because of the export issue, but I went ahead because it looked like a good way to jot stuff down that I might want to remember for my "real" journal. I fire off personal observations all the time to Twitter and don't preserve them, so I didn't see much difference. It turns out I'm enjoying the tagging and storage of this app; for example, I keep a "fuzzies" tag. If something happens that makes me feel good, I jot it down and tag it "fuzzies." When I'm pissed off I can pull up my "fuzzies" entries. If the developers follow through with a good export functionality I'll use the app even more, but it's a superb start and worth the $2.99 (App Store link).