The Collaborative International Dictionary of English v.0.48:

Cheerful \Cheer"ful\ (ch[=e]r"f[.u]l; 277), a.
   Having or showing good spirits or joy; cheering; cheery;
   contented; happy; joyful; lively; animated; willing.
   [1913 Webster]

         To entertain a cheerful disposition.     --Shak.
   [1913 Webster]

         The cheerful birds of sundry kind
         Do chant sweet music.                    --Spenser.
   [1913 Webster]

         A cheerful confidence in the mercy of God. --Macaulay.
   [1913 Webster]

         This general applause and cheerful shout. --Shak.

   Syn: Lively; animated; gay; joyful; lightsome; gleeful;
        blithe; airy; sprightly; jocund; jolly; joyous;
        vivacious; buoyant; sunny; happy; hopeful.
        [1913 Webster]

WordNet (r) 3.0 (2006):

cheerful
    adj 1: being full of or promoting cheer; having or showing good
           spirits; "her cheerful nature"; "a cheerful greeting"; "a
           cheerful room"; "as cheerful as anyone confined to a
           hospital bed could be" [ant: cheerless, depressing,
           uncheerful]
    2: pleasantly (even unrealistically) optimistic [syn:
       cheerful, pollyannaish, upbeat]

Moby Thesaurus II by Grady Ward, 1.0:

143 Moby Thesaurus words for "cheerful":
   Leibnizian, affable, agreeable, airy, amiable, amicable, animated,
   animating, beaming, beatific, beatified, blessed, blissful, blithe,
   blithesome, bright, bright and sunny, buoyant, capering, carefree,
   cheering, cheery, chiliastic, chirk, chirping, chirpy, chirrupy,
   comfortable, compatible, complaisant, congenial, cordial, corky,
   couleur de rose, cozy, dancing, debonair, desirable, dulcet,
   elated, en rapport, encouraging, enjoyable, enlivening, eupeptic,
   euphoric, exalted, exhilarated, exhilarating, exuberant, fair,
   fair and pleasant, felicific, felicitous, fine, flushed,
   flushed with joy, friendly, gay, genial, glad, gladdening,
   gladsome, gleeful, glowing, good, goodly, gracious, grateful,
   gratifying, happy, harmonious, heart-warming, heartening, high,
   homelike, homely, homey, homish, honeyed, hopeful, in good spirits,
   in high spirits, inspiring, inspiriting, intimate, invigorating,
   irrepressible, jaunty, jolly, joyful, joyous, jubilant, laughing,
   leaping, light-hearted, lighthearted, lightsome, likable, lively,
   mellifluous, mellow, merry, millenarian, millennialistic, nice,
   of good cheer, optimistic, peaceful, perfectibilitarian,
   perfectionist, plain, pleasant, pleasing, pleasurable,
   pleasure-giving, pleasureful, purring, radiant, rewarding, riant,
   rose-colored, roseate, rosy, sanguine, sanguineous, satisfying,
   simple, singing, smiling, smirking, snug, sparkling, starry-eyed,
   sunny, sweet, thrice happy, unpretending, upbeat, utopian,
   vivacious, welcome, winsome