Lot 666 of the LOST auction (how's that for Numbers!) was an assortment of props related to the bomb detonated on the Kahana freighter. The purchaser made some of the blocks of C4 available for sale. I purchased one, which came with a copy of the COA.
Although the prop is pretty simplistic (block of styrofoam wrapped in paper and covered with clear tape), it is super-awesome because of its prominence in the Season 4 finale, and its substantial impact on the series.