WordNet (r) 3.0 (2006):

pop in
    v 1: enter briefly; "He popped in for two minutes" [ant: pop
         out]

Moby Thesaurus II by Grady Ward, 1.0:

62 Moby Thesaurus words for "pop in":
   admit, barge in, be admitted, break in, breeze in, burst in,
   bust in, call, come barging in, come breezing in, come busting in,
   come by, come in, come over, creep in, cross the threshold,
   crowd in, drop, drop in, ease in, edge in, enter, gain admittance,
   get in, go in, go into, have an entree, have an in, hop in, infuse,
   inject, inoculate, insert, insinuate, interject, introduce,
   intromit, intrude, irrupt, jam in, jump in, look in, pack in,
   perfuse, press in, push in, put in, set foot in, set in, slip in,
   squeeze in, step in, stick in, stop by, take in, throw in,
   thrust in, tuck in, visit, wedge in, whip in, work in