Mercator Projection, from Google with Google-ly bits removed.

gistfile1.js

var Mercator = (function(){
         /**
          Mercator Projection
          ref: https://developers.google.com/maps/documentation/javascript/examples/map-coordinates?csw=1
        **/
        function MercatorProjection(tileSize) {
            this.tileSize = tileSize || 256;

            this.ONE_DEGREE = Math.PI / 180;

            this.pixelOrigin_ = [this.tileSize / 2, this.tileSize / 2];
            this.pixelsPerLonDegree_ = this.tileSize / 360;
            this.pixelsPerLonRadian_ = this.tileSize / (2 * Math.PI);
        };

        MercatorProjection.prototype.bound = function(value, opt_min, opt_max){
            if (opt_min != null) value = Math.max(value, opt_min);
            if (opt_max != null) value = Math.min(value, opt_max);
            return value;
        };

        MercatorProjection.prototype.radiansToDegrees = function(rad){
          return rad / this.ONE_DEGREE;
        };

        MercatorProjection.prototype.degreesToRadians = function(deg){
          return deg * this.ONE_DEGREE;
        };

        MercatorProjection.prototype.fromLatLngToPoint = function(latLng, zoom, opt_point) {
          var me = this;
          var point = opt_point || [0, 0];
          var origin = me.pixelOrigin_;

          point[0] = origin[0] + latLng[1] * me.pixelsPerLonDegree_;

          // Truncating to 0.9999 effectively limits latitude to 89.189. This is
          // about a third of a tile past the edge of the world tile.
          var siny = me.bound(Math.sin(me.degreesToRadians(latLng[0])), -0.9999,
              0.9999);
          point[1] = origin[1] + 0.5 * Math.log((1 + siny) / (1 - siny)) *
              -me.pixelsPerLonRadian_;

            var n = 1 << zoom;
            point[0] = point[0] * n;
            point[1] = point[1] * n;
          return point;
        };

        MercatorProjection.prototype.fromPointToLatLng = function(point) {
          var me = this;
          var origin = me.pixelOrigin_;
          var lng = (point[0] - origin[0]) / me.pixelsPerLonDegree_;
          var latRadians = (point[1] - origin[1]) / -me.pixelsPerLonRadian_;
          var lat = me.radiansToDegrees(2 * Math.atan(Math.exp(latRadians)) -
              Math.PI / 2);
          return [lat, lng];
        };

        return new MercatorProjection(256);
    })();