The Collaborative International Dictionary of English v.0.48:

activist \activist\ n.
   1. one who is aggressively active on behalf of a cause.

   Syn: militant
        [WordNet 1.5] activist

The Collaborative International Dictionary of English v.0.48:

activist \activist\ activistic \activistic\adj.
   1. 1 advocating a cause or engaged in activism
      [WordNet 1.5]

WordNet (r) 3.0 (2006):

activist
    adj 1: advocating or engaged in activism [syn: activist,
           activistic]
    n 1: a militant reformer [syn: militant, activist]

Moby Thesaurus II by Grady Ward, 1.0:

22 Moby Thesaurus words for "activist":
   ball of fire, beaver, big-time operator, bustler, busy bee, doer,
   eager beaver, enthusiast, go-getter, human dynamo, hustler,
   live wire, man of action, man of deeds, militant, new broom,
   operator, political activist, powerhouse, take-charge guy,
   wheeler-dealer, winner