Peter Knight works with Massimo and Alex and co. at Tinker.it. He’s written some great AVR code, which is useful in Arduino. For example:
Secret Thermometer takes advantage of the ATMega’s internal thermometer. Turns your ‘328-based Arduino into a thermometer with no extra parts.
Secret Voltmeter same idea, but this reads the internal analog-to-digital converter to tell you the Arduino’s supply voltage. Also works on the ATMega168.
He’s also done Cantarino, a speech synthesis engine; Auduino, a granular sound synthesis engine; a DMX library; and more. Check them all out at the tinker.it code repository.