Bert Bril, one of the key developers behind OpendTect made a little SEG-Y zip program to compress SEG-Y files. The program is available for free (released under LGPL) from sourceforge.net.
Bert claims that the program reduces the most common SEG-Y files to at least 50% without a loss that is detectable in the shape of the FFT spectrum. Higher reduction factors (up to 4x) can be achieved by setting the compression level higher. Some deterioration may occur but the effects remain random in frequency. The program is tiny (like 30kb), so that it can easily be sent together with compressed data. The program removes spikes from the data before compressing. Setting the compression level to zero SEG-Y zip effectively becomes a despiking tool and can be used as such.