This is slight variantion on cups-filters' generic PDF printer driver. The original driver provides no means for printing color documents in grayscale. This driver converts all input to grayscale before sending it to the printer, allowing you to conserve that precious color toner.
To install this filter on Debian Wheezy, run
make install
in this directory (as root). The list of Generic printer drivers in the CUPS web interface should have a "Generic PDF Printer (Grayscale)" entry from that point on (provided you have already installed CUPS). If you haven't or you are running a different system entirely, adapt the Makefile to suit your environment first. You can also manually copy pdf2grayscalepdf to a location where CUPS will have it in its shell search path and import the PPD in the CUPS web interface.
Depending on the input document the filter will sometimes fail to work with a
"loadFile failed: temp file: unable to find trailer dictionary while
recovering damaged file"
message.