Stunning design and a clean, intuitive interface (I mean seriously, look at those screenshots on their site) highlight this logbook/journal app, a shining example of the quality of work being done on iPhone applications. I purchased this app after reading an inspiring blog post about keeping a logbook. I have never been great at keeping a journal, and I realized that it was partly because I set the bar too high for content and partly because I don't like carrying extra junk around with me. Momento solves both problems: by simply asking me to record "moments," and by existing on my phone.
It also has the bells and whistles you'd expect from a journaling app. You can set it to import your Twitter/Facebook/Flickr/etc. feeds automatically; you can add tags for people, places, events, and things; you can add photos and star ratings; you can search and export. And did I mention how great it looks? But it's not just eye candy; it is great design in the purest sense of the word: form AND function. It's a joy to use.
I've recently become re-enamored with Camera+ after not really using it much for a while. If you only buy one photo app, this is the one to get. Aptly named, it extends the functionality of the native camera application, adding a composition grid, stability controls, timer, and separate controls for focus and white balance. It also boasts the best stable of filters available for the price. A bargain at 99 cents.
(Also? If Camera+ hadn't been temporarily banned from the App Store when Instagram was released, I don't think you'd even know what Instagram was. And for pure social photo sharing on the iPhone, I think Path is the most beautiful option out there. I wish more people used it.)
Group texting encased in a nice interface. Once you form a group (easily done by choosing from among your contacts or manually entering numbers), anything anyone sends is seen by everyone in the group. The group gets a dedicated number so anyone with a texting plan can participate, but people with the app, which is also available for Android and Blackberry, can choose to receive push notifications instead of SMS and can see the conversation thread. Messages always show the name of the person who set them. Perfect for groups of people on the go who need to stay in touch.