Here are 3 sites which delve into the fascinating world of number stations.
Priyom is an international organization intending to research and bring to light the mysterious reality of intelligence and military communication via shortwave radio: numbers stations. See here for main site: http://priyom.org/ and here for the number stations schedule: http://priyom.org/number-stations/station-schedule.aspx
There is also the "Numbers & Oddities" site which covers the fascinating world of Numbers Stations and all kinds of other odd signals on HF, which can be found here: http://www.numbersoddities.nl/. All their backdated newsletters can also be downloaded, and you may sign up to receive it monthly.
Last but not least is the ENIGMA 2000 (The European Numbers Information Gathering & Monitoring Association), a UK based online group, whose aims are to bring together listeners and enthusiasts who monitor and gather information on 'Number Stations' and other related radio transmissions. Through their Yahoo Group monitors can share their logs, discuss frequencies, thoughts and opinions on this most emotive subject. ENIGMA 2000 maintains the "ENIGMA Control List", the definitive listing of number station identification. Their site can be found here: http://www.apul64.dsl.pipex.com/enigma2000/index.html