We developed a precision combined GPS+GLONASS+SBAS GNSS receiver expansion board for the RasPi called RasPiGNSS (see http://drfasching.com/products/gnss/raspignss.html).
It is based on the NV08C-CSM GNSS chipset, one of the most advanced L1 receivers available from NVS Technologies (http://www.nvs-gnss.com).
Here's how it looks like:
Besides NMEA output this board delivers also raw GNSS data that can be processed with RTKLIB (http://www.rtklib.com), either on the RasPi itself or elsewhere, enabling PPP (precise point positioning) solutions up to decimeter-precision, and even centimeter-precision in RTK (real-time kinematics) mode using a reference station!
Note that this is not your everyday convenience GPS stick - don't buy it if you're just after a longitude/latitude position. It is more suited as a precise measurement device for field survey, robotics, drones or traffic tolling applications.
A commercial service selling a complete device plus server account for RTKLIB postprocessing is available from http://www.pi-gnss.com. We made parts of the software available as open source, as well as the STEP files for 3D-printing the smart case that fits exactly to a PI model A with the RasPiGNSS expansion board. You can download the software and the STEP files here: http://drfasching.com/products/gnss/ras ... ation.html.
Please find more information about RasPiGNSS at http://drfasching.com/products/gnss/raspignss.html!