Sound/Spy Recorder
Fleeting- External reference: com.miragestack.secret.voice.recorder_1.1.4.apk
To me, the killer feature of spy recorder is the ability to trigger a record while the screen is off, by pressing the power button thrice in 2s.
I can get the phone in my pocket, start the recording and put it back without even looking at it.
It vibrates when the record starts and when it ends.
It use same vibration pattern though, and some times I get it wrong and start recording in my pocket ;-).
But still, no other app that I know of provides this feature.