I wish there was an app called "find my bitcoins" as that would be the killer bitcoin app
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I think another killer app would be "watch my bitcoins".
It would notify you when certain bitcoins on the block chain got touched, as a sentinel for your machine being hacked.
It could come with a list of known addresses for sites/people, so when your bitcoins are sent it would say 'xbtc sent to xaddress. That address is known to be mtgox's'
To get the app, you would have to compile a list of 150 addresses you come across and their owners, contributing to the app.