The Collaborative International Dictionary of English v.0.48:

Filler \Fill"er\, n.
   1. One who, or that which, fills; something used for filling.
      [1913 Webster]

            'T is mere filler, to stop a vacancy in the
            hexameter.                            --Dryden.
      [1913 Webster]

            They have six diggers to four fillers, so as to keep
            the fillers always at work.           --Mortimer.
      [1913 Webster]

   2. (Paint.) A composition, as of powdered silica and oil,
      used to fill the pores and grain of wood before applying
      paint, varnish, etc.
      [Webster 1913 Suppl.]

   3. (Forestry) Any standing tree or standard higher than the
      surrounding coppice in the form of forest known as coppice
      under standards. Chiefly used in the pl.
      [Webster 1913 Suppl.]

The Collaborative International Dictionary of English v.0.48:

Filler \Fill"er\, n. [From 1st Fill.]
   A thill horse. [Prov. Eng.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

filler
    n 1: used for filling cracks or holes in a surface
    2: 100 filler equal 1 forint in Hungary
    3: copy to fill space between more important articles in the
       layout of a magazine or newspaper
    4: anything added to fill out a whole; "some of the items in the
       collection are mere makeweights" [syn: makeweight,
       filler]
    5: the tobacco used to form the core of a cigar

Moby Thesaurus II by Grady Ward, 1.0:

91 Moby Thesaurus words for "filler":
   IC analysis, appositive, attribute, attributive, bidet, bush,
   bushing, carriage horse, cart horse, cavalry horse, complement,
   construction modifier, cutting, deep structure, direct object,
   doubling, doublure, draft horse, dray horse, driving horse, facing,
   fill horse, filling, form-function unit, function, gigster, hack,
   hackney, hunter, immediate constituent analysis, indirect object,
   inlay, inlayer, insole, interlineation, jument, lead, leader,
   levels, liner, lining, modifier, mount, object, pack horse,
   packing, padding, palfrey, phrase structure, plow horse,
   pole horse, polo pony, post-horse, predicate, qualifier, ranks,
   remount, rider, riding horse, road horse, roadster, rouncy,
   saddle horse, saddler, shaft horse, shallow structure, slot,
   slot and filler, stalking-horse, strata, structure, stuffing,
   subject, sumpter, sumpter horse, surface structure,
   syntactic analysis, syntactic structure, syntactics, syntax,
   tagmeme, thill horse, thiller, underlying structure, wadding,
   wainscot, wheeler, wheelhorse, word arrangement, word order,
   workhorse