Harminv is a free program (and accompanying library) to solve the problem
of harmonic inversion - given a discrete-time, finite-length signal
that consists of a sum of finitely-many sinusoids (possibly exponentially 
decaying) in a given bandwidth, it determines the frequencies, decay 
constants, amplitudes, and phases of those sinusoids. 

WWW: http://ab-initio.mit.edu/wiki/index.php/Harminv