as seen at freshmeat, and at SIXPACK site: "SIXPACK is a free BibTeX and Reference Manager designed to edit, convert and manage reference files, search and sort bibliographies, import and export many different bibliography types. Sixpack uses the excelent perl package bp by Dana Jacobson to covert between formats." and it reads/writes bibtex, refer, endnote, tip, procite, rfc1807, and plain text along with one-way for several others. written in perl/tk... looks real good.
Stuart writes: "M[n]M is software that
supports processing of national standard MARC MeSH subject authority records for use in medical library online public access catalogs. While designed for use with Innopac M[n]M can also be used to manage MARC MeSH in other OPACs. M[n]M is freely available under the GNU Public License. To download M[n]M or to read more, go to library.med.nyu.edu/mm".
There's also a very informative paper describing M[n]M available here.