The Collaborative International Dictionary of English v.0.48:

Piquant \Pi"quant\, a. [F., p. pr. of piquer to prick or sting.
   See Pike.]
   Stimulating to the taste; giving zest; tart; sharp; pungent;
   as, a piquant anecdote. "As piquant to the tongue as salt."
   --Addison. "Piquant railleries." --Gov. of Tongue.
   [1913 Webster]

WordNet (r) 3.0 (2006):

piquant
    adj 1: having an agreeably pungent taste [syn: piquant,
           savory, savoury, spicy, zesty]
    2: engagingly stimulating or provocative; "a piquant wit";
       "salty language" [syn: piquant, salty]
    3: attracting or delighting; "an engaging frankness"; "a piquant
       face with large appealing eyes" [syn: engaging, piquant]

Moby Thesaurus II by Grady Ward, 1.0:

147 Moby Thesaurus words for "piquant":
   acerbic, acid, acrid, agitating, alluring, appealing, appetizing,
   asperous, astringent, attractive, beguiling, bewitching, biting,
   bitter, blandishing, breathtaking, cajoling, captivating, catching,
   caustic, challenging, charged, charismatic, charming,
   cliff-hanging, coaxing, come-hither, coquettish, cutting,
   disquieting, distracting, disturbing, electric, enchanting,
   encouraging, energizing, engaging, enravishing, enthralling,
   enticing, entrancing, escharotic, exciting, exhilarating, exotic,
   fascinating, fetching, flirtatious, galvanic, galvanizing,
   glamorous, harsh, heady, heart-expanding, heart-stirring,
   heart-swelling, heart-thrilling, hypnotic, impressive,
   inflammatory, interesting, intoxicating, intriguing, inviting,
   irresistible, irritating, jarring, jolting, juicy, keen, lively,
   maddening, mesmeric, mind-blowing, mordant, mouth-watering, moving,
   nose-tickling, overcoming, overmastering, overpowering,
   overwhelming, penetrating, peppery, peppy, perturbing, piercing,
   poignant, prepossessing, prompting, provocative, provoking,
   provoquant, pungent, racy, ravishing, readable, rich, rough,
   rousing, seducing, seductive, severe, sharp, siren, sirenic,
   snappy, soul-stirring, sour, sparkling, spellbinding, spellful,
   spicy, spirit-stirring, spirited, stimulant, stimulating,
   stimulative, stinging, stirring, striking, succulent, suspenseful,
   suspensive, taking, tantalizing, teasing, telling, tempting,
   thought-challenging, thought-inspiring, thought-provoking,
   thrilling, thrilly, tickling, titillating, titillative, trenchant,
   troubling, unsettling, upsetting, vitriolic, vivacious, winning,
   winsome, witching, zesty