If all your sites are within the same domain, but just using subdomains, Bakery is a good solution:

http://drupal.org/project/bakery


Jamie Holly
http://www.intoxination.net 
http://www.hollyit.net
On 9/17/2012 5:21 PM, Disnel Rodríguez Rodrí­guez wrote:
I have several Drupal sites, each one with a different purpose but all of them made for the same organization. But I have a problem since the same user must authenticate himself in every site..., and that is very unconfortable.  Is there any module that allows a user puts his/her credentials in a site and then enter to another site using the same credentials?..., in other words, is it possible put all the sites together with the same authenticaton system (eg. local, LDAP, etc.)? Hope you can help me..., thanks...