Pompidou. Here is the official site. But my favourite description is here (where else?). From the outside, you should guess it must be something related with arts (each colors actually represent something functional).
If you are not so artsy, being around Pompidou is fun enough. There are some free exhibits by the pond, such as La Sirene. Better, you can see tourists acting wild for photograph (i.e. me)
If you have Paris Card, do go in!
