WordWeb Online

1. Full of or showing high-spirited merriment
   "a mirthful laugh";
   - gay, jocund, jolly, jovial, merry
    
2. Arousing or provoking laughter
   "a mirthful experience";
   - amusing, comic, comical, funny, laughable, risible, rib-tickling

See also: humorous, humourous [Brit, Cdn, non-standard], joyous