Louvre HD (US$1.99) is a beautifully done app that takes you on a virtual tour of one of the greatest collections of paintings in the world.
The app provides access to 2,300 paintings from the Louvre in Paris that can be viewed in very high resolution. Louvre HD itself isn't that large (only 76 MB), but requires an internet connection to access the high-resolution images of the paintings.
The works are organized by century and cover 800 different artists. There is a quick search function, and paintings can be saved to your iDevice or mailed to friends. The high-resolution images invite exploration, because you can two-finger zoom and really explore these painting in great detail.
Louvre HD also has a slideshow mode with adjustable speed, and you can even choose some classical music to listen to in the background.