UPDATE: This technique only works up to Concrete5.6.2.1. Starting with Concrete5.6.3, passwords are saved to the database using a different hashing method (which is a good thing), so this trick (which is specific to the old MD5 hashing method) will not work on 5.6.3 or higher.

Here's a single SQL query that resets the admin password:

UPDATE Users SET uPassword = md5('newpassword:PASSWORD_SALT') WHERE uName = 'admin';

Replace newpassword with the new password you want the admin user to have, and replace PASSWORD_SALT with the value defined in your config/site.php file.

I find myself using this technique a lot when copying a live site to my local machine for development.

Credit: This technique is taken from http://www.concrete5.org/documentation/how-tos/developers/i-cant-login/ -- I just posted it here so I can find it more easily in the future :)