In this application note, Atmel describe how to detect zero crossing on an AC waveform using a handful of passive components. This is useful for triggering triacs, as well as the home automation protocol X10.

Remember that there is no isolation between the microcontroller and mains here, so take heed to their notes regarding surge supression.

 http://www.atmel.com/dyn/resources/prod_documents/doc2508.pdf