Today at midnight (from December 21, 2007 at 0h) 9 countries are joining the Schengen space. So with one Schengen visa, you will be able to travel in 24 E.U. countries.

The countries joining are the following: Estonia, Hungary, Latvia, Lithuania, Malta, Poland, Slovakia, Slovenia and the Czech Republic.

Initial Schengen countries were: Austria, Belgium, France, Germany, Greece, Italy, Luxembourg, the Netherlands, Portugal, Spain and since March 2001: Denmark, Finland, Iceland, Norway and Sweden (Switzerland will join in 2008, but recognize Schengen visas).
Initially the lifting of internal immigration controls will involve just land and sea borders, but that will be extended to airports at the end of March 2008.

Ireland and UK are still out of the Schengen space.

Read here: http://news.bbc.co.uk/1/hi/world/europe/7153490.stm