The Collaborative International Dictionary of English v.0.48:

Rueful \Rue"ful\ (r[udd]"f[.u]l), a.
   1. Causing one to rue or lament; woeful; mournful; sorrowful.
      [1913 Webster]

   2. Expressing sorrow. "Rueful faces." --Dryden.
      [1913 Webster]

            Two rueful figures, with long black cloaks. --Sir W.
                                                  Scott.
      [1913 Webster] -- Rue"ful*ly, adv. -- Rue"ful*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

rueful
    adj 1: feeling or expressing pain or sorrow for sins or offenses
           [syn: contrite, remorseful, rueful, ruthful]

Moby Thesaurus II by Grady Ward, 1.0:

86 Moby Thesaurus words for "rueful":
   addicted, affecting, afflictive, aggrieved, anguished, ashamed,
   bitter, bleak, careworn, cheerless, comfortless,
   conscience-smitten, conscience-stricken, deplorable, depressed,
   depressing, depressive, despairing, despondent, discomforting,
   dismal, dismaying, distressful, distressing, doleful, dolorific,
   dolorogenic, dolorous, dreary, dumb with grief, full of remorse,
   grief-stricken, griefful, grieved, grievous, hopeless, in grief,
   joyless, lamentable, lugubrious, miserable, mournful, moving,
   oppressed, painful, pathetic, piteous, pitiable, pitiful,
   plaintive, plangent, plunged in grief, poignant, poor, regretful,
   regrettable, remorseful, repining, ruthful, sad, saddening,
   self-accusing, self-condemning, self-convicting, self-debasing,
   self-flagellating, self-humiliating, self-punishing,
   self-reproaching, shamefaced, shamefast, shameful, sharp, sore,
   sorrowed, sorrowful, sorrowing, sorry, tearful, touching,
   uncomfortable, unhappy about, wistful, woebegone, woeful,
   wretched