Are you fed up of your own country's definitive series, classic, contemporary, even the colonial ones? Know that foreign countries can be interesting too.
Adrian proposed a little tour of Europe, with the Flowers of Germany, for example.
The Plate Number Coil Collectors Club (PNC3) listed the different forms of sale of the United States Forever stamp (reminder).
On the side of the inexhaustible Machin, the incredible Douglas Myall informed that the Royal Mail put a tiny difference in the different forms of sale, that is of printing and making, of the new non-denominated stamps with the new security features. A letter changes everything: to your magnifyer! And do not forget the rest.
27 April 2009 : to help you with the new Machins, the MachinManiacals summarized what is known about the stamps from booklets.