Go to admin/config/system/actions and Create an advanced action, selecting Send an e-mail.
At the bottom are all the tokens available to insert in your text.
(Note: Not all of these tokens are working / maybe incorrect, maybe buggy: http://drupal.org/node/870868
Send e-mail to administrator for new user
New user has signed up [user:mail] [author:mail] [user:name]
Then go to Triggers.