WordNet (r) 3.0 (2006):

take on
    v 1: take on a certain form, attribute, or aspect; "His voice
         took on a sad tone"; "The story took a new turn"; "he
         adopted an air of superiority"; "She assumed strange
         manners"; "The gods assume human or animal form in these
         fables" [syn: assume, acquire, adopt, take on,
         take]
    2: take on titles, offices, duties, responsibilities; "When will
       the new President assume office?" [syn: assume, adopt,
       take on, take over]
    3: accept as a challenge; "I'll tackle this difficult task"
       [syn: undertake, tackle, take on]
    4: admit into a group or community; "accept students for
       graduate study"; "We'll have to vote on whether or not to
       admit a new member" [syn: accept, admit, take, take
       on]
    5: contend against an opponent in a sport, game, or battle;
       "Princeton plays Yale this weekend"; "Charlie likes to play
       Mary" [syn: meet, encounter, play, take on]

Moby Thesaurus II by Grady Ward, 1.0:

222 Moby Thesaurus words for "take on":
   accept, ache, acquire, add, admit, adopt, agonize, air a grievance,
   annex, antagonize, append, appropriate, assume, attack, attempt,
   battle, be livid, be pissed, beat against, beat up against, beef,
   bellyache, bespeak, bitch, bleed, bluster, boil, book,
   breast the wave, brief, brood over, browned off, buck, buckle to,
   buffet, buffet the waves, burn, carry on, chafe, clamor,
   close with, combat, come by, come in for, compete with, complain,
   conduct, contend against, contest, copy, crab, croak, derive,
   derive from, dive into, do, do battle, drag down, draw, draw from,
   embark in, embark upon, employ, endeavor, engage, engage in,
   engage in battle, enter on, enter upon, exercise, fall into,
   fall to, fight, fight against, follow, fret, fret and fume, fume,
   fuss, gain, get, get busy, get cracking, get going, get under way,
   get with it, give a fling, give a go, give a try, give a whirl,
   give battle, go about, go at, go in for, go into, go on, go to it,
   go upon, grapple with, grieve, gripe, groan, grouch, grouse, growl,
   grumble, grunt, have, have a conniption, have at, have coming in,
   hire, holler, hop to it, howl, imitate, infringe, join battle,
   join battle with, join issue, jump to it, kick, labor against,
   launch forth, launch into, lay about, light into,
   lodge a complaint, make a try, make use of, militate against, mock,
   mope, mourn, move into, murmur, mutter, obtain, offer resistance,
   pine, pine away, pirate, pissed off, pitch in, pitch into,
   plagiarize, plunge into, practice, preengage, proceed to,
   prosecute, pull, pull down, pursue, put on, rage, raise Cain,
   raise a howl, raise hell, raise the devil, raise the roof, rant,
   rant and rave, rave, receive, recruit, register a complaint,
   reluct, reluctate, reserve, retain, rival, sail into, secure,
   seethe, set about, set at, set forward, set going, set to,
   set to work, sign on, sign up, sign up for, simmer, simulate,
   sizzle, smoke, smolder, sorrow, specialize in, squawk, start in,
   steal, stem the tide, stew, storm, strike, strive against,
   struggle against, subjoin, superadd, tackle, take, take in,
   take into employment, take over, take to, take up, throw a fit,
   try conclusions, turn to, undertake, use, venture upon, vie with,
   wade into, wage, work at, worry, yap, yelp