When we were at the toy museum the other day, Ebi-kun started playing with this tannguramo made Kumon. One of the staff noticed him playing with it and bought over the full set of cards to go with it, he refused to leave until he had finished the whole lot - 20 cards!
In the set you get a box of wooden pieces and 20 cards with holes cut in them. in the corner of the card it tells you how many wooden pieces are needed to complete the puzzle and the cards get progressively more difficult. The box also has a booklet with even more puzzles to do.
He was so engrossed with it we decided to get him a set, he loves it. I know these types of puzzle are popular in Japan, there is usually a set or two in the shien centres and play centres but I don't know if they are popular elsewhere.
I do have a soft spot for Kumon products, they are usually well made and well thought through.