WordNet (r) 3.0 (2006):

soapbox
    n 1: a crate for packing soap
    2: a platform raised above the surrounding level to give
       prominence to the person on it [syn: dais, podium,
       pulpit, rostrum, ambo, stump, soapbox]

Moby Thesaurus II by Grady Ward, 1.0:

29 Moby Thesaurus words for "soapbox":
   balcony, catafalque, dais, declaim, emplacement, estrade, floor,
   gallery, harangue, heliport, hustings, landing, landing pad,
   landing stage, launching pad, mouth, perorate, platform, podium,
   pulpit, rant, rave, rostrum, stage, step terrace, stump, terrace,
   tribunal, tribune