A garbage collector for C/C++ (Hans Boehm)
Hans Boehm's page on the widely used Boehm-Demers-Weiser conservative garbage collector for C/C++. The Boehm-Demers-Weiser conservative garbage collector can be used as a garbage collecting replacement for C malloc or C++ new. It is also used by a number of programming language implementations that use C as intermediate code.
http://www.hpl.hp.com/personal/Hans_Boehm/gc/

An Introduction to Garbage Collection, Part I
Presents an introduction to garbage collection, presenting the subject in enough detail to show the various tradeoffs and advantages between various techniques and what goes on under the hood in a typical garbage collector.
http://icu.sourceforge.net/docs/papers/cpp_report/an_introduction_to_garbage_collection_part_i.html

Garbage Collection: Automatic Memory Management in the Microsoft .NET Framework
Explains how resources are allocated and managed, then gives a description of how the garbage collection algorithm works. Also discussed are the way resources can clean up properly when the garbage collector decides to free a resource's memory and how to force an object to clean up when it is freed. By Jeffrey Richter, MSDN Magazine.
http://msdn.microsoft.com/msdnmag/issues/1100/GCI/default.aspx

PngEncoder
java png encoder, the fastes java pngencoder around, footprint is just over 5K.
http://objectplanet.com/PngEncoder/

Richard Jones' Garbage Collection Page
Richard Jones wrote Garbage Collection: Algorithms for Automatic Dynamic Memory Management", the first English book concerned solely with garbage collection. Garbage collection page has an FAQ, and a large bibliography.
http://www.cs.kent.ac.uk/people/staff/rej/gc.html

Ramakrishan Venkitaraman's Home Page
Contains source code for networking and distributed system projects. Also contains the source code for the Garbage Collector for the C programming language.
http://www.utdallas.edu/~ramakrishnan/

Factor
Dynamically typed, stack-based language; has optimizing native compiler, automatic memory management, generational garbage collector, collections library, advanced features: higher-order programming, continuations, extensible syntax.
http://factorcode.org/

Parallel and Distributed Garbage Collectors
A parallel and distributed extension of the BDW conservative collector.
http://www.yl.is.s.u-tokyo.ac.jp/gc/

The YAFL Programming Language
Mid-term research project to design, implement an object-oriented language, and several attached tools; major goals: be strict language with inheritance and garbage collector, unique compiler support facility. [Open Source, GPL]
http://www.phidani.be/yafl/

Paul Wilson's Garbage Collection Archive
A collection of papers on garbage collection, memory allocation, and the like from the OOPS Research Group at the University of Texas at Austin. Includes papers from the OOPSLA Workshops on Garbage Collection in 1990, 1991, and 1993.
ftp://ftp.cs.utexas.edu/pub/garbage/