The Collaborative International Dictionary of English v.0.48:

I O U \I O U\, IOU \I.O.U\([imac]" [=o]" [=u]"). [phonetic, from
   I owe you.]
   A paper having on it these letters, with a sum named, and
   duly signed; -- in use in England as an acknowledgment of a
   debt, and taken as evidence thereof, but not amounting to a
   promissory note; a due bill. --Wharton. --Story.
   [1913 Webster]

WordNet (r) 3.0 (2006):

IOU
    n 1: an informal debt instrument; representing `I owe you'

Moby Thesaurus II by Grady Ward, 1.0:

39 Moby Thesaurus words for "IOU":
   CD, MO, acceptance, acceptance bill, bank acceptance, bank check,
   bill, bill of draft, bill of exchange, blank check, certificate,
   certificate of deposit, certified check, check, checkbook, cheque,
   commercial paper, debenture, demand bill, demand draft, draft,
   due bill, exchequer bill, letter of credit, money order,
   negotiable instrument, note, note of hand, paper, postal order,
   promissory note, sight bill, sight draft, time bill, time draft,
   trade acceptance, treasury bill, voucher, warrant