On my recent trip to Chile, I used the following iPhone apps:
Word Lens: Realtime translation. You point the iPhone at a sign or menu and you see live video, but the Spanish words are translated into English. It not only provides a "Wow!" experience, it's actually by far the most useful translator, since it can be used with relatively little fanfare on, for instance, a menu. The words sometimes swim in and out of translation and only the words, not the grammar, is translated, but you definitely get the sense of what's going on.
Photosynth: Panoramic photos. You click "start" and the camera takes a photo, bordered with a dash. As you move the camera around, more photos are added automatically … click click click and a panorama is built up in no time. When you are done, the stitching of the images is completed and you have a panoramic image. The stitching is not perfect (take a look at this quite-poor job of the Gemini South telescope, which I thought would be an excellent candidate, given the strong lines of the telescope's struts), but the image-capture is lightning fast and on landscapes it seems to do a better job.
Camera+: Photography. There are a few things I like about this application over the iPhone's built-in camera app. First, it has a "stabilizer" mode which only takes a photo when your hand has steadied; obviously, you can't use this in all situations, but I prefer it when trying to take a landscape. Second, it has built-in sharing for Facebook. Third, you can crop and do basic adjustments in the app. Pretty simple to use and adds some real functionality.
Trip Journal: Note-taking. A limited, but useful, product that allows you to keep together your photos, notes, and locations. If you actually want to put together a travelogue, you want to organize your stuff by time, which can be difficult if you keeo your notes in one program, your photos in another, and GPS waypoints in a third. Trip Journal ties together these things, but cannot import notes or photos from other applications and has very limited editing capabilities. It turns out, though, that it's export format (.tjz) is just a zip file that contains an XML document and the photo JPEGs, so if you're the type of person who says "Oh! Well, that makes it easy," it's a good product.
Night Stand: Alarm Clock I use this as my alarm clock. Some reviews complain of the alarm not working, but I use this app every day and it work for me. What people may be complaining about is that if you set the alarm clock to a song that is subsequently removed from the phone, the app will not complain (e.g., I have a "smart playlist" that puts highly-rated and "not recently played" music on my phone; if I set Night Stand to wake to some song on that playlist and subsequently sync the iPhone and that song gets removed, Night Stand will silently fail). So instead, I just make sure that the song I choose to wake to is on the phone and won't get bumped (on this trip, I chose to wake to "Drunken Sailor," by the Blaggards. Hey ho and up she rises!)
Tom Tom Chile: Navigation A few years ago, I drove around for 45 minutes in what couldn't have been more than an 8-block area looking for my hotel. I swore that I would never rent a car without turn-by-turn navigation again. This app is probably the most expensive thing I've ever bought for my iPhone, but for me, in a foreign country, with extremely limited Spanish, and no 3G Internet (I actually didn't bring my SIM-card with me on this trip), it was worth it. The street maps seemed quite accurate but were very, very bad about 1-way restrictions, which were extremely common. So "Ms. Garmin" would often be commanding me to move towards an impossible route. But, at some point she'd give up and reroute me through something manageable. I used this with a vent mount that I brought with me, but I neglected to bring an in-car recharger, which would have been a mistake had I not been primarily in the city. The Garmin app sucks power, even when it's in the background (I think what drains the power is the real-time navigation; pressing Clear Route when you don't need it seems to help quite a bit).