My fave ipad app so far...

[ffutures]

Although I'll probably never have a huge amount of use for it, my favourite app so far (and I've even paid £1.49 for it) is Measures HD, which uses the built-in sensors etc. for various possibly useful purposes

Tesla meter - fairly sensitive magnetic field meter
Decibel meter - noise level with graph
Big compass with GPS latitude and longitude
Protractor - not sure what I will use it for
Ruler - ditto
level gauge
Spirit level
Plumb bob
Seismograph - yes, really, very sensitive too
Stop watch
Timers
Metronome

I'm reasonably sure I'll never need most of these functions, but it does seem amazingly cool.