WordNet (r) 3.0 (2006):

drop in
    v 1: visit informally and spontaneously; "We frequently drop by
         the neighbors' house for a cup of coffee" [syn: drop by,
         drop in, come by]

Moby Thesaurus II by Grady Ward, 1.0:

55 Moby Thesaurus words for "drop in":
   admit, barge in, be admitted, break in, breeze in, burst in,
   bust in, call, call on, call upon, come barging in,
   come breezing in, come busting in, come by, come in, come over,
   creep in, cross the threshold, crowd in, edge in, enter,
   gain admittance, get in, go in, go into, have an entree,
   have an in, hop in, insert, intrude, irrupt, jam in, jump in,
   look in, look up, pack in, pay a visit, pop in, press in, push in,
   put in, run in, see, set foot in, slip in, squeeze in, step in,
   stop by, stop off, stop over, take in, thrust in, visit, wedge in,
   work in