What iPhone apps can't you live without? What are your "must-have" iPhone or iPod touch apps—the ones you can't live without? (Figuratively speaking, I hope.) Please omit apps that come pre-installed.

Rules


One app per answer.
Use this format for the first two lines of your answer:
## [app name](link to website)
[App Store](link to appstore)   
Include short description.
Check for duplicates before adding new answers. 
If you find a duplicate, vote it down and encourage its poster to upvote the original entry instead (and remove the dupe).


Please exclude iPad-only apps from here; there's Great iPad Apps for those. 

Disclaimer: I know this type of question is controversial. However, great many people do find that properly implemented and curated CW polls can be useful. (If you are one of those who do not, could you simply consider ignoring this?)
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