The Block Cart, as the name suggests, is a cart upon which one can attach a block. Sort of a cross between a sticky piston and minecart. Why, you ask? This idea is also intended to compliment another idea, the Block Comparator which compares blocks on either side of it and sends a charge forward if they are the same. Even as a stand alone suggestion, a cart that can tote a block around can still have many uses. As it is "sticky" it could travel under a pushed forward generated stone or cobblestone and carry it to where another piston could push it off. An activator rail could also make a block cart detach from it's payload (become momentarily unsticky). I'm sure there are possibilities I haven't even thought of as well. The ability to automatically reposition blocks in the game, quietly, quickly and over large distances opens up a new dynamic which I'm sure Minecrafters would take full advantage of.