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]

WordNet (r) 3.0 (2006):

useful
    adj 1: being of use or service; "the girl felt motherly and
           useful"; "a useful job"; "a useful member of society"
           [syn: useful, utile] [ant: useless]
    2: having a useful function; "utilitarian steel tables" [syn:
       utilitarian, useful]

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



Bouvier's Law Dictionary, Revised 6th Ed (1856):

USEFUL. That which may be put into beneficial practice.
     2. The patent act of congress of July 4, 1836, sect. 6, in describing
the subjects of patents, mentions "new and useful art," and "new and useful
improvement." To entitle the inventor to a patent, his invention must, to a
certain extent, be beneficial to the community, and not be for an unlawful
object, or frivolous, or insignificant. 1 Mason, 182; 1 Pet. C. C. R. 322; 1
Bald. 303; 14 Pick. 217; Paine, 203.