I've made an online heat map generator that you can use by clicking here. Below is an example heat map made with the tool.

The generator allows you to set the size of the grid, paste data from Excel and export to Excel. You can also take a screenshot of the generated heat map if you'd like a JPEG/PNG.

The code is available by viewing the source (it's pure Javascript). The one bit of code that's not available is the Excel export code which is just outputting the HTML with an Excel header (i.e. "Content-type: application/x-msdownload; charset=utf-16").