The Collaborative International Dictionary of English v.0.48:

Needful \Need"ful\ (n[=e]d"f[.u]l), a.
   1. Full of need; in need or want; needy; distressing.
      [Archaic] --Chaucer.
      [1913 Webster]

            The needful time of trouble.          --Bk. of Com.
                                                  Prayer.
      [1913 Webster]

   2. Necessary for supply or relief; requisite.
      [1913 Webster]

            All things needful for defense abound. --Dryden.
      [1913 Webster] -- Need"ful*ly, adv. -- Need"ful*ness,
      n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

needful
    adj 1: necessary for relief or supply; "provided them with all
           things needful" [syn: needed, needful, required,
           requisite]

Moby Thesaurus II by Grady Ward, 1.0:

27 Moby Thesaurus words for "needful":
   called for, cardinal, essential, fundamental, imperative,
   indicated, indispensable, irreducible, irreductible, irreplaceable,
   mazuma, moolah, necessary, needed, ooftish, pelf, prerequisite,
   required, requisite, rhino, rocks, scratch, shekels, smash,
   unforgoable, vital, wanted