Code Generator Kits

BEG - a Back End Generator
BEG is a commercial generator for compiler back ends. It reads a declarative description of a target processor (i.e. SPARC, MIPS, ARM, x86, etc.) and produces a possibly optimizing code generator in form of a C program.
http://www.hei.biz/beg/

Dynamically Targetable Tools Framework (DTTF)
A framework for building dynamically targetable compilers, assemblers and simulators for a wide variety of processor architectures. [Commercial]
http://www.sankhya.com/info/products/tools/dttf.html

ExpertCoder
ExpertCoder is a toolkit for the .NET platform that supports the creation of code generators based on expert systems. Not a code generator-generator but rather a set of libraries useful for writing generators, it is released under the GPL license.
http://expertcoder.sourceforge.net/

froofyJIT
C++ front end for Paolo Bonzini's GNU lightning library for dynamic native code generation. Uses C++'s language facilities to achieve conciseness approximating a `real' assembly language.
http://sourceforge.net/projects/froofyjit/

JBurg: a Bottom-Up Rewrite Machine Generator for Java
Java-based BURG (code emitter generator) which can also be used as a general purpose dynamic programming engine. [Open source, Common Public License]
http://jburg.sourceforge.net/

New Jersey Machine-Code Toolkit
Helps programmers write applications that process machine code: assemblers, disassemblers, code generators, tracers, profilers, debuggers. Toolkit lets programmers encode and decode machine instructions symbolically. Encoding and decoding are automated based on compact specifications.
http://www.eecs.harvard.edu/~nr/toolkit/

vcode
A library for dynamically generating machine code from within a C program. Code is generated using a generic RISC-like virtual instruction set which is then translated into native processor instructions. Currently vcode supports MIPS, SPARC, and Alpha processors.
http://pdos.csail.mit.edu/~engler/pldi96-abstract.html