The Collaborative International Dictionary of English v.0.48:

nifty \nifty\ adj.
   1. Very good; excellent; -- an informal term meaning about
      the same as groovy, sense 1. [informal, 1960's]

   Syn: bang-up, bully, cool, corking, cracking, dandy, great,
        groovy, keen, neat, not bad(predicate), peachy, slap-up,
        swell, smashing.
        [WordNet 1.5]

   2. Stylish; attractive; -- of clothing; as, a nifty outfit.
      [PJC]

WordNet (r) 3.0 (2006):

nifty
    adj 1: very good; "he did a bully job"; "a neat sports car";
           "had a great time at the party"; "you look simply
           smashing" [syn: bang-up, bully, corking,
           cracking, dandy, great, groovy, keen, neat,
           nifty, not bad(p), peachy, slap-up, swell,
           smashing]

Moby Thesaurus II by Grady Ward, 1.0:

107 Moby Thesaurus words for "nifty":
   OK, ace-high, agile, apt, bad, bang-up, bonzer, boss, braw, bully,
   but good, chic, classy, clever, clothes-conscious, considerable,
   cool, corker, corking, cosmopolitan, crackerjack, daisy, dandy,
   dapper, dashing, delicious, dressed to advantage, dressed to kill,
   ducky, elegant, energetic, excellent, exquisite, fab, fine,
   fine and dandy, gear, genteel, good, great, groovy, healthy, heavy,
   hot, humdinger, hunky-dory, in good form, jam-up, jaunty,
   just dandy, keen, knockout, marvy, mean, modish, natty, neat,
   nobby, okay, out of sight, peach, peachy, peachy-keen, posh,
   profitable, quick, recherche, ripping, ritzy, rum, satisfactory,
   satisfying, scrumptious, sharp, slap-up, sleek, smart, smashing,
   smug, snazzy, soigne, soignee, solid, something else,
   sophisticated, spiffing, spiffy, splendid, spruce, spry, stunning,
   style-conscious, stylish, substantial, suitable, super, swank,
   swanky, swell, terrific, tough, tricksy, trig, trim, well-dressed,
   well-groomed, wizard