RefDB is a reference database and bibliography tool for SGML, XML, and LaTeX documents, sort of a Reference Manager or BibTeX for markup languages. It is portable and known to run on Linux, Free/NetBSD, OSX, Solaris, and Windows/Cygwin.
Editor extensions for Emacs (shown here) and Vim
Running a query on the command line
The result of a SRU query displayed as a MODS document