Solution for http://codecombat.com/play/level/gridmancer Relatively simple solution that solves the problem with 33 rectangles. Most of the code is spent handling an alternate grid that makes it easier to reason about tiles.

gistfile1.js

var tileSize = 4;

var makeGrid = function() {
	var grid = [],
		navGrid = this.getNavGrid().grid,
		height = ~~(navGrid.length / tileSize),
		width = ~~(navGrid[0].length / tileSize);
	for (var y = 0; y < height; y++) {
		for (var x = 0; x < width; x++) {
			grid[y] = grid[y] || [];
			grid[y].push(!!navGrid[y*tileSize][x*tileSize].length);
		}
	}
	return grid;
};
makeGrid = makeGrid.bind(this);

var drawGrid = function(grid) {
	grid.rectangles.forEach(drawRectangle);
};
drawGrid = drawGrid.bind(this);

var drawRectangle = function(rect) {
	this.addRect(
		(rect.x * tileSize) + ((rect.width * tileSize) / 2),
		(rect.y * tileSize) + ((rect.height * tileSize) / 2),
		(rect.width * tileSize),
		(rect.height * tileSize));
	this.wait();
};
drawRectangle = drawRectangle.bind(this);

var fillGrid = function(grid) {
	grid.rectangles = [];
	for (var y = 0; y < grid.length; y++) {
		for (var x = 0; x < grid[0].length; x++) {
			if (!grid[y][x]) {
				this.growRectangle(x, y, grid);
			}
		}
	}
};
fillGrid = fillGrid.bind(this);

this.growRectangle = function(x, y, grid) {
	var rect = {
		x: x,
		y: y,
		width: 0,
		height: 0
	}, x2, y2, heightFound, widthFound, toggle;
	while (!(heightFound && widthFound)) {
		if (toggle) {
			rect.width++;
			x2 = rect.x + rect.width - 1;
			for (y2 = rect.y; y2 < rect.y+rect.height; y2++) {
				if (x2 >= grid[0].length || grid[y2][x2]) {
					rect.width--;
					widthFound = true;
					break;
				}
			}
		} else {
			rect.height++;
			y2 = rect.y + rect.height - 1;
			for (x2 = rect.x; x2 < rect.x+rect.width; x2++) {
				if (y2 >= grid.length || grid[y2][x2]) {
					rect.height--;
					heightFound = true;
					break;
				}
			}
		}
		toggle = !toggle;
	}
	for (var y3 = rect.y; y3 < rect.y + rect.height; y3++) {
		for (var x3 = rect.x; x3 < rect.x + rect.width; x3++) {
			grid[y3][x3] = true;
		}
	}
	grid.rectangles.push(rect);
};

var grid = makeGrid();
fillGrid(grid);
drawGrid(grid);
this.say("Delicious. Did it in "+this.spawnedRectangles.length+" rectangles");