RTC: Real-Time Clock
For some projects you might want to have RTC on the board to keep time ticking event if the board is disconnected. I have noticed Sparkfun Real Time Clock Module (BOB-00099): http://www.sparkfun.com/products/99
This was is using I2C interface and I2C is available of R-P final boards.
It uses DS1307 RTC, which already has Linux driver (I2C) in the mainline kernel (/drivers/rtc/rtc-ds1307.c).
This was looks a nice RTC solution and a nice exercise with HW and Linux kernel getting it working.