For a while I've been using the sirlabs.de vocoder plugin:
http://www.sirlab.de/linux/descr_vocoder.html
But it has some problems, notably expecting two inputs but only offering
one output. There was a patch on linuxrockstar which gave it two
outputs, but something seemed a bit lacking, still.
An evening of hacking and here we have it:
http://www.gjcp.net/~gordonjcp/vocoder.tar.bz2
It's hard-set to eight bands, all with the same level. There's a
bandspread control to alter the frequency spacing of the partials, and a
width control to vary the stereo effect where odd bands go to the left
output and even bands go to the right output.
Give it a go and let me know how you get on.
Gordon