Really one of the best toys invented in the last couple of decades would have to be Stikfas. We discovered them at Think Geek, but they’re available lots of places.
They look like the plastic of a model car when you buy them; lots of molded pieces attached together. You break the pieces off and then snap them together to make the figures. This photo does not do them justice; they’re so customizable, you’ll play with them for hours. They come with stickers to decorate the figures, and there are lots of other types of accessories. Weapons, tools, animals, dinosaurs, vehicles, armor, and more.