The team at Dexter Industries ran a successful Kickstarter campaign last year to fund their $30 Arduberry hat which sits atop a Raspberry Pi and allows you to plug Arduino Shields to the RPi environment. The company is now shipping the Arduberry. It can be programmed from the Arduino IDE running on the Raspberry Pi and the RPi programs can communicate with the Arduberry and the Arduino shields using Python or C. The raspberry Pi can be configured to talk to the shields directly. An additional power jack on the Arduberry can also be used to provide additional power to the Arduino shields if necessary.
The Arduberry is compatible with the Raspberry Pi Models B, and B+ and hooks up via the 26-connector on the Raspberry Pi. While most shields can work directly with the Raspberry Pi, some sketches, like those that use analog signals, will need to be modified so that the sketch can relay information to the Raspberry Pi. A tutorial has been created that walks you through how to adapt those sketches.
The design uses an extended header, which is left open for expansion and testing. The Arduberry's contains a microcontroller that is most similar to the Arduino UNO with an Atmega 328 and Arduino UNO bootloader installed and has a shield footprint almost identical to the Arduino UNO. It also incorporates a voltage translator between the Raspberry Pi and the Atmega 328 chip supplies. The design is fully open-source and the details are available of Github if you want to delve deeper.