Sichtwerk GmbH, Appenzell has Altimeter+ (iTunes Link) for the iPhone and iPod Touch in the iTunes App Store for FREE for a limited time. This deal expires May 19, 2012.

This Altimeter determines the height using a digital elevation model. ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) provides digital elevation data with an accuracy of ±10m.
Alternatively, you can also only use GPS for height determination. The GPS altitude is less accurate.

ASTER only delivers the topographical elevation. Example: When ballooning, ASTER does not provide the height of the balloon, but the elevation of the underlying ground.