If you like art, the Artsy site is worth a visit. Not only is it well put together, it has a lot of good content i.e. great art. If you click on an image, you can (usually) zoom into an image and get a proper look at it as well.
Of immediate interest is seeing the shows, both current and previous, listed for a museum. The page for the National Gallery includes a good selection of work from the current show, Inventing Impressionism :
Looking forward to the Goya show later in the year.
Similar to Artsy is the Google Art Project, which also lets you zoom around museum collections around the world. I'm sure you can spend hours on a site like these.