your question is a bit vague....

(edited)

link is broken - sry

did you look at http://dm.ncl.ac.uk/joescully/voltronlib/


http://processing.org/reference/libraries/

or via google maybe : http://code.google.com/p/dataprole/downloads/list

hi,

the simplest thing would be to set up a 3d grid of regular cubic cells (voxels) and check each voxel if it is intersected by the model (its polygons). if so its "filled", otherwise not,... just like pixels.
the problem here is, this can is very slow, and a lot of cells will be empty, which is wasted memory.

so a better way is, to use some smart data-structure, like an octree: http://en.wikipedia.org/wiki/Octree

some months ago i did an octree-implementation in processing that "voxelizes" polygon-meshes imported from an obj-file.
here are some screenshots, and the github-link to the source-code (eclipse-project):
http://thomasdiewald.com/blog/?p=1488

also have a look at the 3d processing libraries:
http://processing.org/reference/libraries/#3d

sometimes you'll find nice features in them that may be useful too.

another starting point might be minecraft or similar projects that use voxels.

thomas