Back when I was taking Computer Graphics 2 at RIT, I made a program just like this. The only difference is that my program had a shittier interface. However, my program let you put blocks anywhere, not just stacking them up, and it let you make the blocks in any color.
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[Edit] And another.
Also, I was trying to write words in shadows, but it went too slow and I said fuck it, and finished watching Gurren Lagann. Fun stuff though.