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.]},
}