A real thermometer for iPhone. No Internet, no GPS needed. RealTemp measures the temperature of the air near the microphone of your phone. It does this by sending out a waveform through the speaker.

PLEASE READ BEFORE BUYING:
iPhone 4 and 3GS needs a 30 sec warmup.
Does not work on iPod Touch.
Only tested on iPhone 4,3G and 3GS.
May not work with protective case.
Works best in silent surroundings.
On some devices the microphone or speaker moves very easily which means that RealTemp needs to be calibrated before every use.
Buy this version to support further development. Later versions may cost more.

Calibration will be necessary as every phone is different. This is easily done by measuring a known temperature (ignore the incorrect temperature that will be displayed while doing this). If you drop your phone or in some other way give it a good shake you will probably need to make a new calibration.

The 3GS and 4 model will require about 30 seconds to "warm up" before an accurate measurement can be done.

Tested from -15 to 30 degrees celsius (0-90 degrees F)
Tested on 3G, 3GS and 4 models only. Earlier models may not work.
Does not work on iPod.
Requires OS 3.0 or later.