One very useful application I have recently used is called
DisplayPad, it allows me to work at my laptop while moving applications such as Mail over to the ipad.
I personally find this much more effective than using the built in ipad apps since more than one application can be on the screen at the same time and I can move my mouse all the way across.
I was originally using
AirDisplay but sadly it seems to take up 100% of my CPU whenever the ipad is on rendering it unusable.
Hopefully they will fix this issue as
AirDisplay generally seems smoother than