Cross Compilers

Amsterdam Compiler Kit (ACK)
An integrated collection of programs designed to simplify the task of producing portable (cross) compilers and interpreters.
http://sourceforge.net/projects/tack/

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

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

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 VGB or no$gmb.
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

SCORE
(Safety Critical Object-oriented Real-time Embedded) ANSI C, C++, Ada, Fortran cross compiler for embedded PowerPC, SPARC, x86 processors based on ANDF (Architectural Neutral Distribution Format) technology.
http://www.ddci.com/products_SCORE.php

SjASMPlus
Modern and 3-pass Z80/R800 Assembly Language Cross Compiler. FreeBSD/DOS/Windows.
http://sjasmplus.sourceforge.net

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/