Michael Gasperi made a homebrew digital (I2C slave) sensor. Creating digital sensors typically requires either some hardware which supports I2C communication, such as PCF8574 digital port expander, or a microcontroller like the Microchip PIC. The later alternative is the most versetile, allowing using all the microcontroller features to interface with the NXT. These include digital input/output, analog input/output, UART (i.e. RS232, needed for instance for most GPS/GSM modules) etc. The pitfall, however, is that one need to program the PIC to behave as a proper “I2C Slave”. In this dedicated page Michael explains how to do this efficiently.

NOTE: Sivan Toledo did a similar job with the TI MSP430 family chips. Take a look here.