Exact diagonalization Studies of Two-dimensional Frustrated   Antiferromagnet Models
  We describe the four kinds of behavior found in two-dimensional isotropic quantum antiferromagnets. Two of them display long range order at T=0: the N\'eel state and the Valence Bond Crystal. The last two are Spin-Liquids. Properties of these different states are shortly described and open questions are underlined. 
C. Lhuillier, P. Sindzingre, J.-B. Fouet