Microchip PIC18FXXX series of microcontrollers. It currently is written using HITECH Software's PICC-18 compiler and uses Microchip's MPLAB Integrated Development Environment (IDE) along with Microchip's In-Circuit Debugger 2 (ICD2).is a application framework - a set of APIs, if you will - written in C for the
It's not an operating system since there's nothing like a kernel. It's more a collection of routines which make the PIC's peripherals easier to use. For example, it includes initialization routines, interrupt handlers, hooks to aplication code, queues for communication peripherals (CAN, UART) and even error handling.
PicCore is available under the GNU General Public License (GPL) v2, despite the fact none of the software modules say so ;)
PicCore User Manual (includes quick intro and overview)
PicCore is currently freely available on the PSAS' CVS servers: http://cvs.psas.pdx.edu/
Directions on how to use CVS to get to the code (rather than via the web interface) can be found on the Software Team Home.