Magna-Tiles are plastic tiles that come in a variety of shapes and have small magnets embedded in the corners so that the tiles stick together enabling children to build 3D shapes from them. There are a number of different set variations, my favorite ones are the translucent colors so that the light can shine through them, but they also come in solid colors too.
I bought these for my oldest when he was about 3, they seemed like a big expense when I first purchased them, but my kids have had so much fun building with them over the years and still play them now 7 years later, the Magna-Tiles have more than paid their way.
They've introduced several new sets since I purchased mine. There is a metropolis set so that you can create your own high-rise city set, which includes doors and stairs. A car expansion set that contains 2 platforms with wheels for children to build upon to create their dream vehicles. Also a glow in the dark set, which includes an LED light so they can write messages on the tiles.