Dcraw is an amazing open source app that decodes many RAW formats to a more usable format for computer manipulation. It’s used by many commercial programs including Google’s Picassa!
It outputs pnm files, but with a little command line magic it can be used to convert a directory of RAW files to Jpeg.
Canon cameras produce RAW files with the extension “.cr2″ so:
for i in *.cr2; do dcraw -c -a -n -h $i | ppmtojpeg > `basename $i cr2`jpg; echo $i done; done
This produces low quality jpeg images alongside your RAW images which makes it useful when browsing the directory with a file manager that doesn’t understand RAW.
You could have the camera produce the low quality jpeg images but why sacrifice the space on your media card?
It goes without saying that this will work on any UNIX platform that Dcraw supports!