The Collaborative International Dictionary of English v.0.48:

Straighten \Straight"en\, v. t. [imp. & p. p. Straighted; p.
   pr. & vb. n. Straighting.]
   1. To make straight; to reduce from a crooked to a straight
      form.
      [1913 Webster]

   2. To make right or correct; to reduce to order; as, to
      straighten one's affairs; to straighten an account.
      [1913 Webster]

   To straighten one's face, to cease laughing or smiling,
      etc., and compose one's features.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Straighten \Straight"en\, v. t.
   A variant of Straiten. [Obs. or R.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

straighten
    v 1: straighten up or out; make straight [syn: straighten,
         unbend] [ant: bend, flex]
    2: make straight [syn: straighten, straighten out]
    3: get up from a sitting or slouching position; "The students
       straightened when the teacher entered"
    4: put (things or places) in order; "Tidy up your room!" [syn:
       tidy, tidy up, clean up, neaten, straighten,
       straighten out, square away]
    5: straighten by unrolling; "roll out the big map" [syn: roll
       out, straighten]
    6: make straight or straighter; "Straighten this post";
       "straighten hair"

Moby Thesaurus II by Grady Ward, 1.0:

25 Moby Thesaurus words for "straighten":
   adjust, arrange, clean up, clear up, correct, disentangle, neaten,
   organize, put in order, rearrange, rectify, reform, rehabilitate,
   reorganize, resolve, settle, sort out, spruce up, straighten out,
   tidy up, uncurl, unkink, unravel, unsnarl, untangle