Personally I like calling them "backpacks", but what we are talking about is a child board, or what Arduino calls a shield.
Cypress Semiconduct makes a family of parts called PSoC. Two of the family members (PSoC 4 and PSoC 5) use ARM processors like Raspberry Pi to power the part. Another family, PSoC 3, uses an 8 bit "8051" MPU. But what makes these things particularly interesting to Raspberry Pi folks is that they present a sophisticated and configurable interface to the real world -- lots and lots of IO and some analog too. What are they used for? In India they run house electric meters; in medical electronics they run everything from breathing machines to tooth brushes. There is one in every Xbox just taking care of buttons, lights, and controls. It's the ultimate project start for automation, control, or user interface.
While these chips are powerful enough to run on their own they have an interesting life running highly configurable child boards for Arduino and, now, RPi. Two companies offer low cost boards that can be worked up as shields or backpacks for Arduino or RPi. FREESOC makes complete PSoC boards with all parts and with prices from about $50 to $100. Schmartboard (no fooling) makes a low cost board that allows people to install different PSoC chips to support different roles.
They have an introductory offer for their new board through Sunday, April 7th that features a PSoC chip soldered in place -- A PSoC5 with the ARM processor -- for the regular price of the board without the chip. Such a deal.
Sites with info include:
Schmartboard: http://www.schmartboard.com/index.asp?p ... dev&id=648