Chrome extension to enable mouse wheel zooming. Click the middle button and use the wheel to zoom, increments of 20%. There's no options yet, and it cancels auto scroll and opening links in new tab with the middle click.

manifest.json

{
    "manifest_version": 2,

	  "name": "Wheel Zoom",
	  "description": "Mouse Wheel Zooming.",
	  "version": "1.0",

	  "permissions": [ 
		  "tabs", 
		  "https://*/*", 
		  "http://*/*" 
	  ],

	  "content_scripts": [{
	    "all_frames": true,
	   	"matches": [
			"http://*/*", 
	   	"https://*/*"
		],

		"js": ["wheelzoom.js"]

	}]
}

wheelzoom.js

zoomWithWheel = false; 
zoomLevel     = 1;
zoomIncrement = 0.2;

window.addEventListener("mousewheel", function( event ) {
    if( zoomWithWheel ) {
        if( event.wheelDeltaY > 0  ) {
            zoomLevel += zoomIncrement;
        }
        if( event.wheelDeltaY < 0 ) {
            zoomLevel -= zoomIncrement;
        }
        document.body.style.zoom = zoomLevel;
        return false;
    }
}, true );


/* 
    Auto scroll listens for this event, so we
    have to intercept it and cancel it
*/
window.addEventListener("mousedown", function( event ) {
    if(event.button == 1) {
      event.preventDefault();
      event.stopPropagation();
    }
}, true);


/*
    Chrome listens for this event to trigger
    open in new tab with middle click, it gets
    a bit confusing to mix and match, so we 
    have to intercept and cancel that also
*/
window.addEventListener("click", function( event ) {
    if(event.button == 1){
        event.preventDefault();
        event.stopPropagation();
    }
}, true);

/*
    And finally, we can stick our own event listener 
    on mouseup and off we jolly well go, eh ?
*/
window.addEventListener("mouseup", function( event ) {
    if(event.button == 1){
        event.preventDefault();
        event.stopPropagation();
        zoomWithWheel = !zoomWithWheel;
    }
}, true);