Field3D is an open source library for storing voxel data. It provides C++ classes that handle in-memory storage and a file format based on HDF5 that allows the C++ objects to be written to and read from disk.
The library is part of Imageworks' open source effort and was launched on Google Code a few weeks ago. The code base is available as of today via SVN (also through Google Code).
More information about the project can be found at: http://code.google.com/p/field3d/
OpenSource at Imageworks: http://opensource.imageworks.com/