I intend to add more here in due course

Also take a look at GNUPIC, http://www.gnupic.org for some freeware PIC tools.

Hi-Tech

Provide a free version of their C compiler PICC-Lite
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.

Sourceboost

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. The free compiler supports 2K of program memory, enough for most projects, especially hobbyist ones. There are a number of different licences available, with prices to match the level required. Readily sold in the UK by Maplin and Farnell.

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.

Updated 02 May 2011