We discuss two-dimensional (2D) free clusters consisting of N cells of equal areas for which the energy reduces to the surface (inter-cell boundary) energy. We search the arrangement (i.e. the topology) of the cells that minimizes the total perimeter of their boundaries (minimal clusters). For particular N's called hexagonal numbers, the minimal cluster has hexagonal inner bubbles arranged in a honeycomb (perfect cluster) and a regular hexagonal periphery along close-packed <01> rows. For other N's, candidates for minimal cluster are: a) perfect clusters with a regular hexagonal <01> periphery but containing one step; b) perfect clusters with a non-regular hexagonal <01> periphery with no steps; and c) defective clusters (with inn<01> periphery. Theoretical arguments and computer simulations with the Surface Evolver program for specific N indicate that, depending on N, all these alternatives are possible for the minimal clusters.