Clearly there are too many good options.
There is also http://drupal.org/project/tokenauth. This would allow you to craft a link for email that would auto-login the user for whichever pages you configured to be accessible via token.
Some details from the module's project page:
Token Authentication provides a token for use in the URL to authenticate users to your site. It only works on paths configured in the settings page.
Originally this module was intended to offer simple authentication for RSS feeds to allow feed readers to pull customized content listings. You can now use it to provide a simple browsing experience of any Drupal content.
Be careful with what paths you expose to Token Authentication, as it is effectively a very simple, very low security password mechanism. You can expose Administration pages through bad configuration.