I intend to add more here in due course
Also take a look at GNUPIC, http://www.gnupic.org for some freeware PIC tools.
Microchip XC8/XC16/XC32
My preferred development tools are the Microchip XC8/XC16 C compilers, with MPLAB X. The integration with MPLAB X (love it or hate it) has made it easier for me to develop projects for various microcontrollers, from the PIC10F200 upto mid-range PIC18F devices to dsPIC models.
Provide a free version of their C compiler
I have been playing with this, it works well and integrates with MPLAB.
The free version supports the following parts:
16F877, 16F877A, 12F675, 12F629, 16F627, 16F627A, 16F684, 16C84, 16F84 and 16F84A
but there are some limitations on memory sizes, typically supporting no more than 2K of RAM.
I first tried this back in 2007, when it was newly released. I tired it again a few years ago and it has really improved and is now a decent development tool in it’s own right.
Sourceboost (now available for free)
They have a BASIC, C and C++ compiler for the Microchip PIC range with an integrated IDE. The device support is good, it covers the 12F, 16F and 18F families.
I have started to use their BoostBASIC compiler for a couple of small projects with some pleasing results.
CCS Inc
Provide a C compiler for a wide range of PIC microcontrollers.
The C2C compiler has evolved considerably since I last looked at it. It is a commercial product, with an IDE and more. Take a look.
http://www.microchipc.com has many useful links, as the name implies.