The Collaborative International Dictionary of English v.0.48:

Harken \Hark"en\ (h[aum]rk"'n), v. t. & i.
   To hearken. --Tennyson.
   [1913 Webster]

WordNet (r) 3.0 (2006):

harken
    v 1: listen; used mostly in the imperative [syn: hark,
         harken, hearken]