WinAVR  
   A suite of executable, open source software development tools for the Atmel AVR series of RISC microprocessors hosted on the Windows platform. It includes the GNU GCC compiler.
   http://winavr.sourceforge.net/

GameBoy Developer's Kit (GBDK)  
   A set of tools that enable to develop programs for the Nintendo GameBoy system, either in C or in assembly. GBDK includes a set of libraries for the most common requirements and generates image files for use with a real GameBoy or with an emulator like VG
   http://gbdk.sourceforge.net/

GCC Toolchain for MSP430  
   Port freeware GNU tools to msp430 architecture. Cross compiler for msp430.
   http://mspgcc.sourceforge.net/

Quetzalcoatl  
   Cross compiler for 6502 processors, runs on Linux, Win32; can compile and link programs written in subset of ANSI C, Assembly, 1983 UPL language, or any mix thereof; can output programs for Commodore VIC-20, 64.
   http://www.kdef.com/geek/vic/quetz.html

TML2  
   cross compiler tools for Z80 target.
   http://home.swipnet.se/~w-68269/tml2/tml2.htm

6502 Cross-Development Languages and Tools  
   List of programming languages targeting the 6502 microprocessor (with an emphasis on Commodore 8-bit computers), which are hosted on another system. A separate list of native Commodore languages is also available.
   http://www.npsnet.com/danf/cbm/cross-development.html

ASxxxx Cross Assemblers  
   Series, with companion linker, coded in C, for microprocessor series: 6800(6802/6808), 6801(hd6303), 6804, 6805, 68HC08, 6809, 68HC11, 68HC12, 68HC16, 8051, 8085(8080), z80(hd64180), H8/3xx, DS8xCxxx, 6500.
   http://shop-pdp.kent.edu/ashtml/asxxxx.htm

Amsterdam Compiler Kit (ACK)  
   An integrated collection of programs designed to simplify the task of producing portable (cross) compilers and interpreters.
   http://www.cs.vu.nl/vakgroepen/cs/ack.html