The Collaborative International Dictionary of English v.0.48:

Helpful \Help"ful\, a.
   Furnishing help; giving aid; assistant; useful; salutary.
   [1913 Webster]

         Heavens make our presence and our practices
         Pleasant and helpful to him!             --Shak.
   -- Help"ful*ly, adv. -- Help"ful*ness, n. --Milton.
   [1913 Webster]

WordNet (r) 3.0 (2006):

helpful
    adj 1: providing assistance or serving a useful function [ant:
           unhelpful]

Moby Thesaurus II by Grady Ward, 1.0:

112 Moby Thesaurus words for "helpful":
   accommodating, accommodative, advantageous, agential, agentival,
   agentive, agreeable, aidful, aiding, appropriate, assisting,
   attentive, auspicious, banausic, beneficial, benevolent, bon,
   bonny, brave, braw, bueno, capital, caring, cogent, commendable,
   commodious, complaisant, conducive, considerate, constructive,
   contributory, cooperative, delicate, effective, elegant,
   employable, estimable, excellent, expedient, facilitating, fair,
   famous, favorable, favoring, fine, fitting, forwarding, friendly,
   functional, furthersome, good, good for, goodly, grand, handy,
   healthy, heedful, indulgent, instrumental, intermediary, kind,
   laudable, lenient, mediating, mediatorial, mindful,
   mindful of others, ministerial, ministering, nice, noble, obliging,
   of general utility, of help, of service, of use, pleasant,
   positive, practical, pragmatic, pragmatical, productive,
   profitable, promoting, proper, propitious, reassuring, regal,
   regardful, remedial, royal, salutary, serviceable, skillful,
   solicitous, sound, splendid, subservient, supportive, sympathetic,
   tactful, therapeutic, thoughtful, tolerant, toward, usable, useful,
   utilitarian, valid, valuable, very good, virtuous