WordNet (r) 3.0 (2006):

slow down
    v 1: lose velocity; move more slowly; "The car decelerated"
         [syn: decelerate, slow, slow down, slow up,
         retard] [ant: accelerate, quicken, speed, speed
         up]
    2: become slow or slower; "Production slowed" [syn: slow,
       slow down, slow up, slack, slacken]
    3: cause to proceed more slowly; "The illness slowed him down"
       [syn: slow, slow down, slow up]
    4: reduce the speed of; "He slowed down the car" [syn:
       decelerate, slow down] [ant: accelerate, speed,
       speed up]
    5: become less tense, rest, or take one's ease; "He relaxed in
       the hot tub"; "Let's all relax after a hard day's work" [syn:
       relax, loosen up, unbend, unwind, decompress, slow
       down] [ant: tense, tense up]

Moby Thesaurus II by Grady Ward, 1.0:

151 Moby Thesaurus words for "slow down":
   abate, allay, alleviate, arrest, assuage, attemper, backpedal,
   backwater, bank the fire, block, blunt, bog down, boycott, brake,
   bridle, chasten, check, clip the wings, confine, constrain,
   contain, control, cool, cool off, curb, curtail, curtail expenses,
   cut back, cut corners, cut down, damp, dampen, de-emphasize,
   deaden, deadlock, decelerate, decrease, delay, detain, diminish,
   dompt, downplay, drag, draw rein, dull, ease, ease off, ease up,
   enjoin, extenuate, go on strike, go out, govern, guard, halt,
   hang up, hinder, hold, hold at bay, hold back, hold fast, hold in,
   hold in check, hold in leash, hold out, hold up, impede, inhibit,
   keep, keep back, keep from, keep in, keep in check,
   keep under control, keep within bounds, lag, lay,
   lay under restraint, lenify, lessen, let down, let up, let up on,
   lighten, lock out, lose ground, lose momentum, lose speed,
   make late, mire, mitigate, moderate, modulate, obstruct, obtund,
   palliate, pare down expenses, picket, play down, prohibit, pull,
   pull in, qualify, reduce, reduce the temperature, reef, rein,
   rein in, relax, restrain, retard, retrench, revolt, roll back,
   set back, shut it down, sit down, slack, slack off, slack up,
   slacken, slow, slow up, smother, snub, sober, sober down, soften,
   stall, stay, stifle, stop, straiten, strike, stymie, subdue,
   suppress, take a reef, take in sail, tame, temper, throttle down,
   tone down, tune down, unbend, underplay, unlax, unwind, walk out,
   weaken, withhold