Simplenote is one my favorite iOS apps. At its premise, it’s a note taking (in plain text) app that syncs to Simplenote’s server. From there, you can sync it to a wide range of other apps to be able to access them.
As you can see, its a very simple interface. It’s laser focused on being the best (and fastest) way to input and sync plain text files to all of your devices. Simplenote has created a great database of other apps that allow you to edit your text files from your desktop (PC and Mac) and android devices. You can see that list here (http://simplenoteapp.com/downloads/)/ Simplenote also offers a web interface (https://simple-note.appspot.com/) to allow you to edit your notes from the web.
Simplenote, while unbelievably simple, offers some more advanced features such as tagging and sharing of notes. Sharing is probably my favorite feature, though. My wife and I have a shared note titled ‘Grocery list’. During the week, we both will add grocery items we think of and regardless of who goes to the store, the list is always available.
Simplenote is free in the App store (http://simplenoteapp.com/downloads/itunes.html) and it’s a universal app (native on iPhone and iPad). It has ads, but it can be unlocked for a one time purchase of $4.99. They also offer a subscription option of $19.99/year that offers creating notes via email, RSS support, and premium technical support.