Seoul Museum of Art, Seosomun
Across the street from the plaza is a famous palace, but I'll come back to that after I visit the Seoul Museum of Art.
Overall, the Seoul Museum of Art is enjoyable, and the main exhibitions are worth the minor admission fee. For those visiting the city on a limited schedule, it's worth the time only if you're a big modern art fan, and if you're visiting the palace across the street. Of course, an interesting temporary exhibition can make it that much better. There are several other modern art museums scattered across the city, but I wasn't able to visit any of them on my trips due to planning issues, though I expect that I'll make it to a few more on my next visit to Seoul... whenever that is.