The Collaborative International Dictionary of English v.0.48:

slither \slith"er\ (sl[i^][th]"[~e]r), v. i. [Cf. G. schlittern,
   LG. schliddern. See slide.]
   To slide; to glide. [Prov. Eng.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

slither
    v 1: to pass or move unobtrusively or smoothly; "They slid
         through the wicket in the big gate" [syn: slither,
         slide]

Moby Thesaurus II by Grady Ward, 1.0:

133 Moby Thesaurus words for "slither":
   amble, avalanche, barge, bowl along, bundle, clump, coast,
   coasting, crawl, creep, drag, droop, flit, flounce, flow, flowing,
   fly, foot, footslog, gait, gallop, glide, gliding, glissade,
   glissando, halt, hippety-hop, hitch, hobble, hop, ice-skate, jog,
   jolt, jump, landslide, landslip, limp, lock step, lumber, lunge,
   lurch, lurk, mince, mincing steps, pace, paddle, peg, piaffe,
   piaffer, plod, prance, prowl, rack, roll, roller-skate, sail,
   sailing, sashay, saunter, scuff, scuffle, scuttle, shamble,
   shuffle, sideslip, sidle, single-foot, skate, skateboard, skating,
   ski, skid, skiing, skim, skip, skitter, slather, sled, sledding,
   sleigh, slick, slidder, slide, slide down, sliding, slink, slip,
   slippage, slipping, slithering, slog, slouch, slowness, snake,
   sneak, snowslide, snowslip, stagger, stalk, stamp, steal, step,
   stomp, straddle, straggle, stride, stroll, strolling gait, strut,
   stump, subsidence, swagger, sweep, sweeping, swing, tittup,
   toboggan, tobogganing, toddle, totter, traipse, tread, trip, trot,
   trudge, undulate, velocity, waddle, walk, wamble, wiggle, wobble,
   worm