As I have written many times, I am a huge 3rd party application fan and can't help loading up my original iPhone with applications to try out. One of the latest applications that I find very useful for the iPhone is craigsphone. craigsphone brings craigslist to your iPhone and unlike the other craigslist applications on the iPhone App Store you can actually create and post an ad right from your iPhone on the go. You can take a photo with your iPhone and include it in your ad so there is no reason to create another post without a photo.
You can browse by category or even search within subcategories (such as Computers & Tech within For Sale). Searches are nice and easy with options for title searches, posts with images, and price minimums and maximums. The UI is very nice and easy and it looks like this iPhone application is a better choice than even using the web version on your PC.
You can create your post right on the iPhone and the UI for this is also quite good and well designed.
There is also a tab for Featured posts if you just want to browse around and check out some fun posts. The "nearby" features are only supported in the San Francisco Bay area and Manhattan, with plans to expand in the future. They are also planning to launch this application on the Android and BlackBerry platforms.
Next Mobile Web also developed my favorite Netflix application, PhoneFlix, and a few other applications. I would love to see them create a Redbox application since there is nothing available yet and I use this service all the time.