Base 10 Blocks
The Base 10 Blocks are essentially an extension of the Cuisenaire rods idea. Available in unit cubes, ten rods, hundred flats, and thousand cubes, the Base 10 blocks are sized so that ten units will fit in the same space as a ten rod, ten ten rods will cover a hundred flat, and ten hundred flats will stack into the size of a thousand cube. Thus, they provide a hands-on way to understand and visualize the denary (i.e. base 10) number system.
In order to match the Cuisenaire rods that we already use, I ordered unit cubes in natural wood (the closest color to white), and the ten rods in orange.
I have made my own cards to go with these blocks (see picture), based on an idea in Natural Structure by Edward and Nancy Walsh. Using blank 3x5 index cards, I made cards reflecting the value of each place in the decimal system from units to thousands. Thousands are written on the full-width cards, hundreds on 3/4-width, tens on half-width, and units on 1/4-width cards. The child can match the cards to the manipulatives, and then combine them to form the number.
At first, we use the cards to simply identify the value of large numbers. Then, we add numbers together and see how this can "make" new tens. This has been the most difficult step for us. It seems that once they get this concept, extending it to the higher place values is relatively simple.
We have found the Base 10 blocks helpful in clarifying place value concepts.