Drupal - Administrator notification on user registration

First, go to /admin/config/system/actions. In the section "Create an advanced action", choose "send an email" and click on "create". It takes you to a page where you need to enter a recipients email address, (enter the Administrator's email address here), subject and message. Click Save.

Second, go to /admin/modules and enable the Trigger module.

Third, go to /admin/structure/trigger/user. In the Section "Trigger: After creating a new user account" Choose "Send e-mail" in the select box. Click Assign.

Example Message:

A user has created an account.

Details:
 - Username: [user:name]
 - Email: [user:mail]
 - Time: [user:created:long]

Activate their account by going here: [user:edit-url]

Although this is an old question, I just found it in a search, and found that the suggested approach, though excellent, does not work with Tokens.

If you want the administrator to be sent an email that contains some or all of the fields that you have attached to your user account: eg something like:

[account:field-contact-name] has created an account. Details submitted: [account:field-business-name]

Use Rules module which offers a nice simple tokenised email approach.

Once you have installed Rules, from /admin/config/workflow/rules, add a new rule.

1) Add an event : After saving a new user account

2) Add an action: Send Mail

3) In the message body, choose your tokens from the Replacement Patterns dropdown and save your new Rule.

4) test, and rejoice at how lovely Rules is.