If you use levmar in your published work, you should include a reference/acknowledgement to this site using the following bibtex entry or equivalent.
@misc{lourakis04LM, author={M.I.A. Lourakis}, title={levmar: Levenberg-Marquardt nonlinear least squares algorithms in {C}/{C}++}, howpublished={[web page] \verb+http://www.ics.forth.gr/~lourakis/levmar/+}, year={Jul. 2004}, note={[Accessed on 31 Jan. 2005.]}, }