Mark Crosbie has written some C libraries for the PIC to control a serial LCD and a Motor Mind B module, and placed them under the GPL.
Antonio Todobom also has an LCD library. His is written in assembly instead of C.
James Cameron (a.k.a Quozl) has several Pic Projects and Libraries. The 12C509 stack math library for example, is part of his Temperature Sensor Project. Or you can check out his Alarm Clock Project for his stack library.
Mike Gore has created PICFLOAT, an extensive floating point library for the MidRange PIC Microcontrollers. PICFLOAT supports IEEE 32bit and 40bit formats and features over 300 macros for nearly every possible floating point operation!