Jury-rig MouseEvent script to return opposite values for all 3 axis of the mouse wheel. Feel free to edit it as you wish if you want to invert only specific directions or make it work only on specific domains.
This script WON'T change scroll direction on pages NOT controlled by JS (majority of sites out there). The main purpose of this script is to change scroll direction on Google Maps and similar sites, which ARE controlled by JS.
This script were tested in Google Chrome only. Most likely it won't work as expected in Firefox. You may need to wrap code into a 'SCRIPT' tag and append it to a page to make it work there.