The Hand and Malt Brewing Company makes some of the best craft beer in Korea. They have a taproom in an out-of-the-way location behind Sinchon or Ewa Stations.
They offer their full line-up of beers, available for drinking inside or outside on their sixth-floor patio. There are several location around the countries selling their beers. These range from small hamburger shops to large pubs.
You can also visit their brewery out in Gyunggi-do, Namyang-ju (35 minutes from Seoul). They offer tours, but you need to book. Check out their website for more details.