The Collaborative International Dictionary of English v.0.48:

Frequently \Fre*quent*ly\, adv.
   At frequent or short intervals; many times; often;
   repeatedly; commonly.
   [1913 Webster]

WordNet (r) 3.0 (2006):

frequently
    adv 1: many times at short intervals; "we often met over a cup
           of coffee" [syn: frequently, often, oftentimes,
           oft, ofttimes] [ant: infrequently, rarely,
           seldom]

Moby Thesaurus II by Grady Ward, 1.0:

43 Moby Thesaurus words for "frequently":
   a lot, again and again, as a rule, chronically, commonly,
   continually, customarily, day after day, day by day,
   every so often, generally, habitually, in many instances,
   inveterately, many a time, many times, many times over, most often,
   much, not infrequently, not seldom, oft, often, often enough,
   oftentimes, ofttimes, ordinarily, over and over, persistently,
   recurrently, recurringly, regularly, repeatedly, repetitively,
   routinely, several times, time after time, time and again,
   times without number, unseldom, usually, whenever you wish,
   year after year