Weighted Pressure Plates are upcomming blocks in Minecraft 1.5. There Are 2 types of Weighted Pressure Plates, a light one and a heavy one. A Light one gives a redstone signal with a length of 1 if there lay 1 to 4 blocks on it, a signal with a lenght of 2 if there lay 5 to 8 blocks on it etc. A Heavy one gives a redstone signal with a lenght of 1 if there lay 1 to 42 blocks on it, a signal with a lenght of 2 if there lay 43 to 85 blocks on it and a redstone signal with a lenght of 3 if there lay 85 to 128 blocks on it etc.
You can make the light one by placing 2 gold ingot next to each other in the crafting grid, and the heavy one by placing 2 iron ingot next to each other in the crafting grid.