After installing Drupal 7 we get User login block on left sidebar. But what if you disabled the User login block and need to log in, you can visit the login page on:

http://yoursite.com/?q=user
[/codefilter_code]

or, if clean URLs are enabled, use
http://yoursite.com/user
[/codefilter_code]

Note: Use the literal word user, not your username or user id.

Reference: