Here is a list of useful iPod Touch/ iPhone Applications for Writers that I've found in the App Store. There are a ton more but I'm only listing the ones the are for free.
The WriMoDemon was made for Nanowrimo but it can be used ANY time of the year. Use it to keep track of your progress for your WIP (daily word count, percentage of completion, days left). When you're done press the reset button and all setting will go back to zero.
Note: You don't copy and paste your work on it. All you do is write the # of words you have written.
Merriam Webster Dictionary
What is a writer without their tools: words. I use this app all the time to look up new words I don’t know. I try to use as much vocabulary as possible because the more words you have in your noggin the more tools you have at hand while you’re writing.
Note: this app also includes the word of the day, so everyday you can learn a new word.
The Story Tracker helps you keep track of the submissions you’ve made for the book you’re querying for. It's really only useful for those who are currently submitting to agents.
Note: it's not the full version but it does just about the same thing.
One Word Lite
One word helps writers keep track of their daily progress. It also tells you how many words you should aim for each day in order to reach your target word count.
Note: It's just like WriMoDemon except it has pretty pictures and it doesn't show the percentage of completion. I personally prefer WriMoDemon than this.
The Writer's Hat
The Writer's hat can help you come up with really random ideas for prompts. Some of them are actually pretty interesting.