WordNet (r) 3.0 (2006):

memo
    n 1: a written proposal or reminder [syn: memo, memorandum,
         memoranda]

Moby Thesaurus II by Grady Ward, 1.0:

24 Moby Thesaurus words for "memo":
   adversaria, aide-memoire, annotation, chit, directive, docket,
   entry, footnote, item, jotting, marginal note, marginalia, memoir,
   memorandum, memorial, minutes, notation, note, notice, register,
   registry, reminder, scholia, scholium