Naver Maps 
Google Maps do not work too well in Korea. As an alternative, we recommend Naver Maps. The map is also connected to Korea’s major search engine, Naver, providing customer reviews and links to the official website, if any, when looking up a restaurant, café, or popular attraction.
App │ Android, iOS (Korean, English, Japanese, Chinese)
Subway Korea 
Subway Korea is a simple metro map that offers real-time subway schedules along with all the basic information you need for taking the subway in all major cities Korea: Seoul and the metropolitan area, Daejeon, Daegu, Busan, and Gwangju.
App │ Android, iOS (Korean, English, Japanese, Chinese)
Kakao T
Taxis are reasonably priced and abundantly available in Seoul, but the language barrier often makes it difficult to catch a taxi on streets. Kakao T operates much like Uber. You provide your current location and your desired destination, then you click the "Request Taxi" button. For more details on how to use the app, this video may be useful.
App │ Android, iOS (Korean, English, Japanese)
