The Montparnasse Tower (Tour Montparnasse) is a boring 689ft tall skyscraper in the upper eastern corner of the 15th Arrondissement. But, it's a great place to look out from the top of. After it was completed in 1972, Paris banned skyscrapers from the city center. It is said (and rightly so) the view from the top of the tower is the best in Paris because you can't see the tower. It costs 10 1/2 € to go to the top, and the view is breathtaking.

The towers official website is here, and a wikipedia article can be found here.