WordNet (r) 3.0 (2006):

ad lib
    adv 1: without advance preparation; "he spoke ad lib" [syn: ad
           lib, ad libitum, spontaneously, impromptu]

Moby Thesaurus II by Grady Ward, 1.0:

119 Moby Thesaurus words for "ad lib":
   a beneplacito, a discretion, ad arbitrium, ad libitum, ad-libbing,
   al piacere, arrangement, at choice, at pleasure, at short notice,
   at sight, at will, be caught napping, be surprised,
   be taken unawares, be unprepared, by ear, caught napping,
   caught off balance, caught short, cook up, dash off,
   disqualification, do offhand, extemporaneous, extemporaneously,
   extemporaneousness, extemporarily, extemporariness, extemporary,
   extempore, extemporization, extemporize, extemporized, fake,
   go off half-cocked, haphazard, hasty, have no plan, impromptu,
   improvisate, improvisation, improvisatorial, improvise, improvised,
   improvision, improviso, incapability, incompetence, jury-rig,
   jury-rigged, knock off, lack of preparation, lash up, make up,
   makeshift, measure, nonpreparation, nonpreparedness, off the cuff,
   off the hip, off-the-cuff, offhand, on sight, out of hand,
   planlessness, play by ear, playing by ear, precipitate,
   pro tempore, rough-and-ready, scrap the plan, snap, stopgap,
   strike off, surprised, taken aback, taken by surprise,
   taken unawares, temporary arrangement, temporary measure,
   throw off, throw together, toss off, toss out, tripped up,
   unarranged, unbegun, unconcocted, uncontrived, undeliberated,
   undevised, unfitness, unfittedness, unforced, unhatched, unmade,
   unmanufactured, unorganized, unplanned, unpremeditated, unprepared,
   unpreparedness, unprimed, unqualification, unqualifiedness,
   unreadiness, unready, unrehearsed, unstudied, unsuitability,
   unsuitableness, unsuitedness, vamp, vulnerability, whip up,
   whomp up, wing it, without coercion