The Collaborative International Dictionary of English v.0.48:

Useful \Use"ful\, a.
   Full of use, advantage, or profit; producing, or having power
   to produce, good; serviceable for any end or object; helpful
   toward advancing any purpose; beneficial; profitable;
   advantageous; as, vessels and instruments useful in a family;
   books useful for improvement; useful knowledge; useful arts.
   [1913 Webster]

         To what can I useful!                    --Milton.
   [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

117 Moby Thesaurus words for "useful":
   advantageous, advisable, agential, agentival, agentive, aidful,
   appropriate, assisting, auspicious, banausic, becoming, befitting,
   beneficial, benevolent, bon, bonny, brave, braw, bueno, capital,
   cogent, commendable, commodious, conducive, congruous,
   constructive, contributory, convenient, decent, desirable,
   effective, elegant, employable, estimable, excellent, expedient,
   facilitating, fair, famous, favorable, favoring, feasible,
   felicitous, fine, fit, fitten, fitting, forwarding, fructuous,
   fruitful, functional, furthersome, gainful, good, good for, goodly,
   grand, handy, happy, healthy, helpful, instrumental, intermediary,
   kind, laudable, likely, mediating, mediatorial, meet, ministerial,
   ministering, nice, noble, of general utility, of help, of service,
   of use, opportune, pleasant, politic, positive, practicable,
   practical, pragmatical, productive, profitable, promoting, proper,
   propitious, recommendable, regal, remedial, right, royal, salutary,
   seasonable, seemly, serviceable, skillful, sortable, sound,
   splendid, subservient, suitable, therapeutic, timely,
   to be desired, toward, usable, utilitarian, valid, valuable,
   very good, virtuous, well-timed, wise, worthwhile