Musees

Louvre
You can't die before have seen this one, that is, live of course
http://www.louvre.fr/

Gemäldegalerie Berlin
The website is not so quiet slick, temporarily offline (?), but the museum is fantastic!
http://www.smpk.de/

Metropolitan Museum of Art
5000 years of art, love at first sight!
http://www.metmuseum.org/

Guggenheim Museum
Visiting this one is obligatory, the website nice but sticky (edit: it got better lately)
http://www.guggenheim.org/