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In the example shown here, builder Leg-O-Matic has designed a rather nimble rig in a jaunty pose. If you click through his Flickr stream you'll see that a full minifigure fits inside the suit acting like a suit of armor.
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There are a wide variety of styles that fit into the 'hardsuit' category. There's even a Flickr photostream completely dedicated to the little buggers.
If you ever feel your minifig need that extra 'umph' to take out an adversary, or to take a trip down to the store for eggs. Try your hand at crafting one of these babies.
(Pictures from Leg-O-Matic's Flickr stream)