The bibutils program set interconverts between various bibliography
formats using a common MODS-format XML intermediate. For example, one
can convert RIS-format files to Bibtex by doing two transformations:
RIS->MODS->Bibtex. By using a common intermediate for N formats, only 2N
programs are required and not N^2-N. These programs operate on the
command line and are styled after standard UNIX-like filters.

WWW: http://www.scripps.edu/~cdputnam/software/bibutils/