Arduino and Raspberry Pi are long time used in many projects. Problem is that its a bit tricky to connect Arduino to Pi in convenient way. You need to make cable adapter or so. This is why one of the first Raspberry Pi volunteers Gert van Lo (this is where GertDuino name came from) decided to build a dedicated Arduino that would connect to Raspberry Pi conveniently and have even more features as well.
GertDuino comes with Two microcontrollers. Standard Atmega328 that handles Arduino functionality. Another is Atmega48 which brings more additional I/Os, handles battery backed RTC and IRDA interface. The board also comes with RS232 level converter and it can be used by any of microcontrollers including Raspberry Pi itself. Additionally there are two buttons and 6 LEDs. More information at element14.