gistfile1.rb

class Array
  def pieces( count )
    indexes = self.piece_indexes( count )
    indexes.map{ |index| self[*index] }
  end

  def piece_indexes( pieces_count )
    chunk_size = self.size / pieces_count
    chunk_size = 1 if chunk_size < 1

    pieces_count = self.size if pieces_count > self.size

    indexes = []
    (0..pieces_count - 1).each do |p|
      start_index = p * chunk_size
      indexes << [start_index, chunk_size]
    end
    return indexes
  end
end

# usage
a = [1,2,3,4,5,6,7,8,9,10]
a.pieces(2) #[[1,2,3,4,5],[6,7,8,9,10]]
a.pieces(5) #[[1,2],[3,4],[5,6],[7,8],[9,10]]

@saterus just schooled us all "Just stay in Enumerable"

def pieces(count)
  self.each_slice(self.count / count).take(count)
end

The take(count) at the end I added to remove the extra collections caused by irregular pieces like pieces(3)