Christophe Couvreur's Octave Toolbox
has been a favorite of mine since the good old days of FTP file swapping. I'm an acoustician by trade and training, so I've often needed to perform octave and third-octave frequency analysis of my time series data. Octave analysis provides frequency content of signals over logarithmically expanding frequency bands (unlike the linear spacing of FFTs), and is commonly used in analysis vibration and acoustic data. This submission flawlessly implements the ANSI standard for digital octave and third-octave filtering in an easy-to-use suite of MATLAB functions.
Note that Octave requires the Signal Processing Toolbox, which you should have anyway if you are doing frequency analysis. :)