Small Device C Compiler v2.9.0 is out (since 22th march 2009), it's a freeware, retargettable, optimizing ANSI - C compiler that targets the Intel 8051, Maxim 80DS390 and the Zilog Z80 based processors. Contiki (a small but modern operating system) for Amstrad CPC was compiled with SDCC. BTW, last version of contiki (v2.3.0) is out since the 27th june 2009.

You will find on Tom & Jerry's site 2 ASM sources (maxam) for compressing Amstrad CPC binaries (better than pucrunch and bitbuster) : Exomizer et Aplib, coming from the PC world. He found them in a forum thread of Amstrad.ESP (spanish site).

Blassic is a basic language interpreter for windows, linux and Yopy PDA. It can interpret Amstrad CPC basic, I didnt try it though.