I'm into indie and alternative music a lot and while visiting Korea for 2 weeks I attended a concert there of 소규모-아카시아밴드 (Sogyumo Acacia Band). They played at 상상마당 Live Hall and I uploaded a movie of one of the songs on Youtube.
Nell. You probably already know this group but it's the only indie band I know. They are still active with only releasing their lastest album last year. My favourite song of their's is A.S which is on the album Healing Process.
타루 (From "The Melody," another indie group)
There's also some good fusion music that breaks out of the pop mold:
m-flo (he's ethnic Korean, but sings in Japanese, sometime mixes in some Korean & English)
Humming Urban Stereo