Le Centre Pompidou
Le Centre Georges Pompidou more commonly referred to as le Centre Pompidou, or just the Pompidou Center is a great modern art museum located in the fourth arrondissement. The museum is known more for its crazy exterior than the actual artwork it houses; the building's design is meant to expose the things that are normally hidden in the construction of buildings, so all the elements like the plumbing and beams and elevators are on the outside, and they're painted bright colors to draw attention to them. It makes for a really eye-grabbing (some might say eye-straining) effect, and the building really stands out whenever you're wandering around the Marais and other surrounding neighborhoods; nothing else in Paris looks like the Pomidou Center, and so even when you just see a sliver of it in the distance through two other buildings, you know exactly what it is.
The collections in the museum are really impressive, and seem to change drastically every time I visit. I think the Tate Modern in London is the only modern art museum I've enjoyed as much as or more than the Pompidou. My favorite bits from the permanent collection at the Pompidou are a dozen or so Picassos; it really blows me away to see these paintings that I've seen in books all my life and suddenly be standing so close to the real thing.
When visiting the Pompidou, keep in mind that there are two entrances: there's the large main entrance in front, at the bottom of a wide, sloping place (always full of street artists, muscians, and various other characters and performers), and another smaller entrance all the way around in the back of the building. The back entrance often has less of a wait, but sometimes the line is just as long as out front. In any case, it's worth taking a walk around the building to check before comitting to a line. If you enter through the back, you'll be upstairs, so head right down to the ticket windows on the ground floor.
Both the entrances in the front and back are just to get you through security, and then once you're inside you can buy your tickets. The ticket lines seems to move a lot faster than the lines for security.
To enter the collections, you have to go up a series of escalators that are contained in a big, zigzagged plexiglass tube that snakes its way up the side of the museum. Make sure to go all the way to the top, even if you're not visiting the galleries up there, to see an amazing view of Paris, with lots of recognizable landmarks. This is a perfect place for a photo before you enter the galleries.
Possibly of interest, there's a café located up on the mezzanine of the museum, and it's a nice place to take a break, read, check e-mail, or just enjoy the ambience. If you're just getting a coffee or tea it's not as insanely overpriced as most museum cafeterias, but beware of high prices and mediocre food quality if you're thinking of getting a snack. There's free wifi in the café, and there are also a few wall sockets around that you can use if you're lucky enough to get one.
The very top floor of the museum houses a semi-fancy restaurant called Le Georges, which this starving writer/artist had the pleasure of sampling once long ago, on his accountant's dime. This place is a riot because it's decorated in a really over-the-top modern art style, with big curvy alien-esque pods and stark white, geometric, angular furniture; almost like a caricature of a snobby, artsy restaurant in a film. And I mean that in the nicest way possible! The view of Paris from the restaurant is absolutely stunning as well.
Related junk from Cool Stuff in Paris:
Museum of Salvador Dalí's surrealist drawings and sculptures.