The National Imagery and Mapping Agency (NIMA -- a U.S. agency within the Department of Defense) has released the source code and select binaries for a product called Geotrans (ver. 2.0.3). This product is built as a stand alone application for converting coordinates between various coordinate systems and datums. Geotrans works similar to a calculator and has a rudimentary ability to process batch files. Unfortunately, it doesn't build under GNU/Linux (wants Motif and some g++ errors...).
In order to make this functionality more widely accessible [to programmers], I, Eric G. Miller, have cobbled together a GNU style build environment for creating shared and static libraries out of the Datum Transformation and Coordinate Conversion and Engine components. This system uses the commonly available tools AutoMake, AutoConf and LibTool. The original sources have not been modified, though NIMA makes no warranty or endorsement about this product (see Source_Code_Disclaimer). One should program toward the Engine interface, as it provides a nice abstraction to the individual conversion routines.
I've named the library geotrans (typically libgeotrans), as I couldn't tell what NIMA would have called the combination of libdtcc and libengine.
This library is similar to the PROJ4 library, which is commonly available. I can't say which is better (as far as the conversions are concerned), though the nomenclature for referring to coordinate systems, datums and ellipsoid is a bit more consistent with this library.
The documents below were converted to Adobe's Portable Document Format from the supplied Word 2.0 documents since most free MicroSoft Word translators don't handle 2.0 documents (and who wants that cruft anyway!). It may be that a few characters are screwed up -- I guess even Word97 has trouble reading Word 2.0!
Have fun!