The simplest way to let clients edit pages

You can try the Front End Editor plugin that allows administrators to edit content in-place. If you don’t like that in particular try 8 WordPress Plugins that enable posting and editing from the front-end Logging in is still a must. You can login from front as well, though.
See:

  1. http://devilsworkshop.org/wordpress-login-form-front/
  2. http://pippinsplugins.com/creating-custom-front-end-registration-and-login-forms/
  3. http://www.wprecipes.com/add-a-login-form-on-your-wordpress-theme
  4. see how http://core.trac.wordpress.org/browser/tags/3.3.1/wp-login.php WordPress does it

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