Check out this station’s eFM at 101.3 FM. It’s entirely English programming with lots of interesting radio shows.
AFN: http://afnkorea.net/
The American Forces Network has two radio networks – both AM and FM. News, music and common interest radio, all in English! Try 102.7 FM or 1530 for AM services.
The Korean Broadcasting System is one of the most popular radio networks in Seoul, offering KBS Radio 1, 2 and 3 (all in Korean).
Another large radio network throughout Korea, offering popular radio shows to the listening public.
MBC: http://www.imbc.com/
Arguably the more popular Korean radio network, offering news, music and talk shows (in Korean, of course).