The Collaborative International Dictionary of English v.0.48:

Infrequently \In*fre"quent*ly\, adv.
   Not frequently; rarely.
   [1913 Webster]

WordNet (r) 3.0 (2006):

infrequently
    adv 1: not many times; "in your 1850 church you not infrequently
           find a dramatic contrast between the sumptuous
           appointments of the building itself and the inhuman
           barrack-like living conditions in the church room" [ant:
           frequently, oft, often, oftentimes, ofttimes]

Moby Thesaurus II by Grady Ward, 1.0:

48 Moby Thesaurus words for "infrequently":
   at infrequent intervals, at intervals, brokenly, by catches,
   by fits, by jerks, by skips, by snatches, disconnectedly,
   discontinuously, exceptionally, exiguously, fitfully, haphazardly,
   hardly, hardly ever, here and there, in places, in spots,
   intermittently, irregularly, little, meagerly, not often,
   now and then, occasionally, on occasion, per saltum, piddlingly,
   randomly, rarely, scantily, scarcely, scarcely ever, scrimpily,
   seldom, skimpily, skippingly, sparsely, sparsim, sporadically,
   spottily, stingily, uncommonly, unfrequently, unoften, very seldom,
   willy-nilly