The kids got a set of Kapla building blocks for Christmas. There is exactly one kind of block, 120 mm by 24 m by 8 mm (120 mm is about 4 1/2 inches). They look too simple to be much fun, but after getting addicted I realized that the simplicity is what's great about them. You can put them together in endless combinations, and they are precision-cut so you can build really tall without them getting wobbly.
I used to think Lego was the ultimate in amazingly versatile building blocks, but I've changed my mind. Kapla planks can be placed in any dimension (there's no up down or sideways) and they don't interlock, so pieces can be placed at any angle. They are too big for a baby to swallow so they're safe in a house or day-care center with children of various ages. They aren't the cheapest toy out there, about $65 for a box of 200, but they are well made and could last generations.
Kapla, from France, was the first to market with these blocks but an American company has put out a copy called Keva Planks which they claim are better and cheaper. Citiblocs is a third player that is usually a good bit cheaper than the other two. The dimensions and build quality are the same. The prices vary, so you can compare prices on kapla, keva, and citiblocs below. All three sets are for 200 pieces. If you decide to buy online, I'd appreciate you clicking through one of the amazon links below.