On Monday 2/9 Veronica Belmont posted on Twitter asking her followers to tweet the 3 iPhone/Touch applications that they could not live without. There are currently over 5oo replies, the results are interesting. See for yourself, go to http://twitter.com use the search link at the bottom of the page and then search "#3apps".
Veronica posed the question, such that a majority of the replies do not include games, you get a sense of the productivity/social/communication applications that iPhone users are using. In my opinion,cutting through the junk apps the clutter the "top tens" on the App store. I think the results are probably skewed with regards to twitter applications, since the survey was conducted exclusively with twitter users. Other than that, To Do applications are big: Things, Remember The Milk (RTM) & Omnifocus all have a presence; Facebook is on a lot of lists, again many people see a real value with Evernote!
I found it worthwhile to browse the replies.. looking for Apps I may have overlooked, you may too!