The Collaborative International Dictionary of English v.0.48:

Haunt \Haunt\ (h[aum]nt; 277), v. t. [imp. & p. p. Haunted; p.
   pr. & vb. n. Haunting.] [F. hanter; of uncertain origin,
   perh. from an assumed LL. ambitare to go about, fr. L. ambire
   (see Ambition); or cf. Icel. heimta to demand, regain, akin
   to heim home (see Home). [root]36.]
   1. To frequent; to resort to frequently; to visit
      pertinaciously or intrusively; to intrude upon.
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            You wrong me, sir, thus still to haunt my house.
                                                  --Shak.
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            Those cares that haunt the court and town. --Swift.
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   2. To inhabit or frequent as a specter; to visit as a ghost
      or apparition; -- said of spirits or ghosts, especially of
      dead people; as, the murdered man haunts the house where
      he died.
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            Foul spirits haunt my resting place.  --Fairfax.
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   3. To practice; to devote one's self to. [Obs.]
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            That other merchandise that men haunt with fraud . .
            . is cursed.                          --Chaucer.
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            Leave honest pleasure, and haunt no good pastime.
                                                  --Ascham.
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   4. To accustom; to habituate. [Obs.]
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            Haunt thyself to pity.                --Wyclif.
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WordNet (r) 3.0 (2006):

haunting
    adj 1: continually recurring to the mind; "haunting memories";
           "the cathedral organ and the distant voices have a
           haunting beauty"- Claudia Cassidy [syn: haunting,
           persistent]
    2: having a deeply disquieting or disturbing effect; "from two
       handsome and talented young men to two haunting horrors of
       disintegration"-Charles Lee

Moby Thesaurus II by Grady Ward, 1.0:

19 Moby Thesaurus words for "haunting":
   continuous, cyclical, ever-recurring, frequent, incessant,
   indelible, nagging, obsessive, periodic, persistent, reappearing,
   recurrent, recurring, returning, revenant, thematic, thick-coming,
   ubiquitous, unforgettable