Question of the day: What is the one mobile app you can't do without?
The great thing about smartphones is that they have eliminated the need for additional technology; in my case, the 'ol alarm clock. My smart phone IS my alarm clock and I cannot live with out the alarm clock app!
Evernote is super useful for tracking ideas, web pages, articles, pictures, recipes...and syncs between my desktop, laptop, ipad and iphone.
Linked in app and calendar app through Google for my Droid
HootSuite for iPhone .. Manages all my channels for me and for my SM clients ...
Dropbox saves many trees.
(and Yelp, of course)
Love this question. Evernote.