As far as Minecraft mods go, FramedBlocks is unfairly good. The only thing stopping me from wacky builds made of impossible materials was that sometimes I need a slab, ok? Or some stairs. And they don't make Grass Blocks in those shapes. Well now they can! Now I am limited mostly by imagination, and possibly by whether I can remember a vanilla block's name.
This isn't news but I was explaining how it worked to another friend (another vote in its favor: it's conceptually very simple!) and gosh I just love the power to place weird shapes willy-nilly. Every fun Lego shape is now mine.