The CGAL library
The Computational Geometry Algorithms Library is an open source C++ library of efficient and reliable geometric algorithms. This includes support for Triangulated Surface Mesh Simplification. It uses an edge collapse techique that works on oriented 2-manifold surface with support for holes and arbitrary genus.

Home Page: http://doc.cgal.org/latest/Surface_mesh_simplification/

The GLOD library
GLOD represents an extremely lightweight approach to Geometric Level of Detail toolkits. You will find that GLOD is not another scene graph library. Instead, GLOD is designed to closely mesh with the standard OpenGL programming model in such a way that using GLOD should be just like using standard OpenGL vertex arrays.

Home Page: http://www.cs.jhu.edu/~graphics/GLOD/

The GTS Library
GTS stands for the GNU Triangulated Surface Library. It is an Open Source Free Software Library intended to provide a set of useful functions to deal with 3D surfaces meshed with interconnected triangles. The code is written entirely in C with an object-oriented approach based mostly on the design of GTK+. Careful attention is paid to performance related issues as the initial goal of GTS is to provide a simple and efficient library to scientists dealing with 3D computational surface meshes.

Home Page: http://gts.sourceforge.net

The MT Package by Leila De Floriani et al.
The Multi-Tesselation (MT) Package comes from Leila De Floriani, Paola Magillo, and Enrico Puppo of the University of Genova. It is a dimension-independent package for the representation and manipulation of spatial objects as simplicial complexes at multiple resolutions. It is supplied as a C++ library with supporting documentation and demos.

Home Page: http://www.disi.unige.it/person/MagilloP/MT/

Progressive Meshes Maya Plugin by Erik Pojar
Erik has written a Maya Plugin that implements a progressive mesh scheme using quadric error metrics. Additionaly, the user can supply custom weights (using Maya's Artisan interface) to control the aggressiveness of the simplification in different parts of the model. Full source code is provided under LGPL.

Home Page: http://www.pojar.net./ProgressiveMesh/

QSlim by Michael Garland
This package contains two components, the MixKit library and the SlimKit collection of surface modeling tools. QSlim is the primary application in the SlimKit collection and contains all the actual surface simplification code. QSlim can be built as both a command-line only "qslim" program as well as an interactive "qvis" program. Both programs should compile on pretty much any Unix system, as well as Windows 9x/NT.

Home Page: http://graphics.cs.uiuc.edu/~garland/software/qslim.html

ROAM implementation by Mark Duchaineau
This is the implementation source code from one of the authors of the original ROAM paper: ROAMing Terrain: Real-time Optimally Adapting Meshes. The ROAM algorithm uses two priority queues to drive split and merge operations that maintain continuous triangulations built from pre-processed bintree triangles. The web site below provides implementation notes for those wishing to create ROAM-like optimizers.

Home Page: http://www.cognigraph.com/ROAM_homepage/

ROAM demo by Bryan Turner
This is the demo code for the Gamasutra article, Real-Time Dynanmic Level of Detail Terrain Rendering with ROAM. ROAM stands for Real-time Optimally Adapting Meshes and is a popular LOD technique for terrain. The code is in C++ with a MS Visual C++ project file and depends upon the GLUT DLL. A Win32 binary is included.

Home Page: http://www.gamasutra.com/features/20000403/turner_01.htm

RSimp by Dmitry Brodsky and Ben Watson
A simplification algorithm that works by refining a very crude initial approximation. Extremely fast, linear for fixed output size. Good output quality, once the output model is larger than a few hundred polygons. Reads ply models. Shouldn't be any platform dependencies. This code is made freely available for non-commercial use only. See home page for commercial users.

Home Page: http://www.cs.nwu.edu/~watsonb/school/research/rsimp.html

SemiSimp by Gong Li and Ben Watson
Semisimp is a semiautomatic simplification tool built ontop of Michael Garland's qslim algorithm. Reads ply models. The code is in C++, developed for the IRIX and Linux platforms, and uses the Motif library. This code is made freely available for non-commercial use only. Commercial users should see the web site for details.

Home Page: http://www.cs.northwestern.edu/~watsonb/projects/semisimp/

Simplification Envelopes by Jon Cohen, Amitabh Varshney, and Greg Turk
Simplification envelopes is a method for automatically generating level-of-detail hierarchies. The user specifies a single error tolerance, the maximum surface deviation of the simplified model from the original model, and a new, simplified model is generated. The output model is generated by making successive modifications to the original model, always making sure that the resulting surface lies between the two envelope surfaces.

Home Page: http://www.cs.unc.edu/~geom/envelope.html

SOAR by Peter Lindstrom and Valerio Pascucci
SOAR (Stateless One-pass Adaptive Refinement) is a 3D terrain engine for view-dependent out-of-core visualization of large terrain surfaces. It is an implementation of the recent terrain work of Lindstrom and Pascucci at LLNL. The code is distribued as ANSI C and is free for non-commercial purposes.

Home Page: http://www.cc.gatech.edu/~lindstro/software/soar/

Out-of-Core Surface Reconstruction by Bolitho, Kazhdan, Burns, and Hoppe
Source code is available for this work on multilevel streaming for out-of-core surface reconstruction. This involves a Poisson-based scheme for reconstructing surfaces from huge collection of scanned points.

Home Page: http://www.cs.jhu.edu/~bolitho/Research/StreamingSurfaceReconstruction/
Also See: http://research.microsoft.com/en-us/um/people/hoppe/

TerraVision by Martin Reddy, Yvan Leclerc, and Lee Iverson
TerraVision is 3D terrain visualization system that lets you browse massive terrain datasets over the Web. It uses a coarsed-grained quadtree-based level of detail system to support streaming of blocks of terrain from different servers around the Web and fuses these into a single coherent representation of the planet. The source code is released under the Mozilla Public License and hosted on SourceForge.

Home Page: http://www.tvgeo.com/
Source Code: https://sourceforge.net/projects/terravision

VDSlib by David Luebke
VDSlib is an open source software package that implements view-dependent simplification, a geometric level-of-detail algorithm for managing the complexity of polygonal scenes for interactive 3-D graphics. Unlike traditional approaches, which compute discreet levels of detail for each object in a preprocess, VDSlib computes a data structure from which the desired level of detail can be dynamically extracted at run-time.

Home Page: http://vdslib.virginia.edu/

VTK by Kitware Inc.
The Visualization Toolkit (VTK) is an open source software system for 3D computer graphics, image processing, and visualization used by thousands of researchers and developers around the world. VTK includes an implementation of the Schroeder mesh decimation algorithm for topology-preserving simplification.

Home Page: http://public.kitware.com/VTK/

Virtual Terrain Project (VTP)
The VTP site is maintained by Ben Discoe with the goal to foster the creation of tools for easily constructing any part of the real world in interactive, 3D digital form. This includes Open Source implementations of several terrain LOD algorithms. The Demo Data is also on this CD-ROM. Ben asks that you request the VTP source code from him at webmaster@vterrain.org so that he can keep track of the number of users.

Home Page: http://www.vterrain.org/