The Collaborative International Dictionary of English v.0.48:

Plop \Plop\ (pl[o^]p), v. i. [imp. & p. p. Plopped (pl[o^]pt);
   p. pr. & vb. n. Plopping.] [Imitative.]
   1. To fall, drop, or move in any way, with a sudden splash or
      slap, as on the surface of water.
      [Webster 1913 Suppl.]

            The body plopped up, turning on its side. --Kipling.
      [Webster 1913 Suppl.]

The Collaborative International Dictionary of English v.0.48:

plop \plop\ v. t.
   1. to drop (something) so that it makes a plopping sound;
      especially, to drop with the sound of something falling
      into water. [WordNet sense 1, 2]
      [WordNet 1.5]

   2. To set down quickly, so as to make a sound; -- used often
      with the reflexive; as, He plopped himself into the sofa.
      [WordNet sense 3]

   Syn: plank, flump, plonk, plunk, plump down, plunk down,
        plump.
        [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

Plop \Plop\, n.
   Act of plopping; the sound made in plopping.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

plop
    adv 1: with a short hollow thud; "plop came the ball down to the
           corner of the green" [syn: plop, plunk]
    n 1: the noise of a rounded object dropping into a liquid
         without a splash
    v 1: drop something with a plopping sound
    2: drop with the sound of something falling into water
    3: set (something or oneself) down with or as if with a noise;
       "He planked the money on the table"; "He planked himself into
       the sofa" [syn: plank, flump, plonk, plop, plunk,
       plump down, plunk down, plump]

Moby Thesaurus II by Grady Ward, 1.0:

111 Moby Thesaurus words for "plop":
   abruptly, bang, blubber, boil, boil over, bubble, bubble over,
   bubble up, burble, cast, cave, cave in, chuck, clap, dash, decline,
   dive, droop, drop, effervesce, exactly, fall, ferment, fizz,
   fizzle, fling, flop, flop down, flump, flump down, flush, founder,
   full, give way, go down, guggle, gurgle, hastily, hiss, hurl,
   impetuously, impulsively, kerplunk, lapse, like a flash,
   like a thunderbolt, lower, nose-dive, of a sudden, on short notice,
   parachute, pitch, plank, plop down, plumb, plummet, plump, plunge,
   plunk, point-blank, pop, pounce, pounce on, pounce upon,
   precipitantly, precipitately, precipitously, precisely, right, sag,
   seethe, set, settle, settle down, sharp, simmer, sink, sink down,
   skin-dive, sky-dive, slap, slouch, slump, slump down, smack,
   smack-dab, sound, spang, sparkle, square, squarely, startlingly,
   stoop, straight, submerge, subside, sudden, suddenly, surprisingly,
   swag, swoop, swoop down, take a header, throw, thrust, toss,
   unawares, unexpectedly, without notice, without warning, work