Metaballs
For a more modern implementation of metaballs, featuring better graphics and a massive speed improvement, take a look at Metaballs II.
Description:
This program uses the "Marching Cubes" algorithm to render "blobby objects". These are described simply as a position and a radius in space. The inverse square gravity formula is used to calculate the influence of each ball on each cube vertex. An isosurface can then be drawn through the scalar field.
Requirements:
No Extensions required.
References:
Fast metaballs, by Andreas Jonsson. http://www.angelcode.com/dev/metaballs/metaballs.asp
An Overview of Metaballs/Blobby Objects, by Matthew Ward. http://www.siggraph.org/education/materials/HyperGraph/modeling/metaballs/metaballs_mward.html
When the Blobs Go Marching Two by Two, by Jeff Lander. http://www.gamasutra.com/features/20000523/lander_pfv.htm
Polygonising a scalar field, by Paul Bourke. http://local.wasp.uwa.edu.au/~pbourke/modelling/polygonise/