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AskLocal, for posting messages associated with locations on the map. Doesn't seem like a big deal at first, then the possibilities hit you when you realize you can limit who can see/reply to your message based on their physical distance from your message location.
 
Twitter for iPhone. Not only is it the most stable and reliable iPhone app, but it introduced tons of new innovative UI concepts like pull-to-refresh that were adopted later by many other iPhone apps.

I would tend to agree, however last night, this app let me down several times, and closed randomly, loosing my tweets. (Didn't save them to drafts). Apart from these issues, its great, just not happy with recent findings.
 
Do you mean most used or most fun? Cause im addicted to Angry Birds, but Id be lost without the National Rail App..
 
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