The Collaborative International Dictionary of English v.0.48:

Frugal \Fru"gal\, a. [L. frugalis, fr. frugi, lit., for fruit;
   hence, fit for food, useful, proper, temperate, the dative of
   frux, frugis, fruit, akin to E. fruit: cf. F. frugal. See
   Fruit, n.]
   1. Economical in the use or appropriation of resources; not
      wasteful or lavish; wise in the expenditure or application
      of force, materials, time, etc.; characterized by
      frugality; sparing; economical; saving; as, a frugal
      housekeeper; frugal of time.
      [1913 Webster]

            I oft admire
            How Nature, wise and frugal, could commit
            Such disproportions.                  --Milton.
      [1913 Webster]

   2. Obtained by, or appropriate to, economy; as, a frugal
      fortune. "Frugal fare." --Dryden.
      [1913 Webster]

WordNet (r) 3.0 (2006):

frugal
    adj 1: avoiding waste; "an economical meal"; "an economical
           shopper"; "a frugal farmer"; "a frugal lunch"; "a sparing
           father and a spending son"; "sparing in their use of heat
           and light"; "stinting in bestowing gifts"; "thrifty
           because they remember the great Depression"; "`scotch' is
           used only informally" [syn: economical, frugal,
           scotch, sparing, stinting]

Moby Thesaurus II by Grady Ward, 1.0:

107 Moby Thesaurus words for "frugal":
   Lenten, Scotch, Spartan, abstemious, ascetic, austere, budget,
   canny, careful, chary, cheap, cheeseparing, conservational,
   conservative, conserving, discreet, dwarfed, dwarfish, easy,
   economic, economical, economizing, economy, exiguous, forehanded,
   frugal to excess, impoverished, inexpensive, jejune, labor-saving,
   lean, limited, low, low-priced, manageable, meager, mean, measured,
   meticulous, miserly, moderate, modest, money-saving, narrow,
   negligible, niggardly, nominal, overfrugal, paltry, parsimonious,
   penny-pinching, penny-wise, penny-wise and pound-foolish,
   penurious, piddling, poor, preserving, provident, prudent,
   prudential, puny, reasonable, restrained, saving, scamping, scant,
   scanty, scrawny, scrimp, scrimping, scrimpy, sensible, shabby,
   shoddy, skimp, skimping, skimpy, slender, slight, slim, small,
   sober, spare, sparing, starvation, stingy, stinted, stinting,
   straitened, stunted, subsistence, temperate, thin, thrifty,
   tight-fisted, time-saving, token, too frugal, unexpensive,
   unnourishing, unnutritious, unwasteful, wary, watered, watery,
   within means, worth the money