The Palais du Luxembourg was built for Marie de Medici in the early 17th century and is now home to the French Senate. Most visitors see it from the surrounding Luxembourg Gardens, but the palace itself can be visited by reservation on limited open days.
It is worth planning ahead if you are interested in French political history or want to see one of the Left Bank's major monuments from the inside. Otherwise, the best view is from the garden's central axis, with the palace behind the pond and formal plantings.