WPReady

The plugin works similar as shortcodes with important differences:
<ul>
<li>It looks for any tag in the post with the following format: {cf your_custom_field_name=your_custom_field_value}</li>
<li>For any tag found, it create a custom field for that post.</li>
<li>The tag is erased from the post.</li>
</ul>
<h4>How to use</h4>
Type in anywhere in a post: 
{cf your_custom_field_name=your_custom_field_value}
Examples:
<ul>
<li>{cf thumbnail=http://wordpressready.com/wp-content/uploads/oneringtorulethemall.jpg}</li>
<li>{cf posticon=/wp-content/uploads/posticon.ico}</li>
<li>{cf my_custom_color=#22fb30}</li>
</ul>
This three examples written inside the post will create three custom fields with respectives values 
when you publish the post. 
After publishing, the tag inside the post are deleted. No other information is erased.
<h4>Notes</h4>
<ul>
<li>Because the way the plugin works, it will run fine with ANY desktop blogger(Windows Live Writer, Scribefire, Qumana, w.Bloggar, Zoundry 
, Thingamablog, Blod Desk, Flock, Post2Blog, Bleezer, you name it)</li>
<li>The plugin can be used even inside the wordpress editor (though what would it be the point?)</li>
<li>You can use any number of custom field for post. </li>
<li>Reserved characters: ‘{cf’ and ‘}’,because it conflicts with the delimiters.</li>
<li>After the post, the tags are automatically erased.</li>
<li>If the custom field is already present, its value will be replaced.</li>
</ul>

Text to Custom Field

Allows adding custom fields from ANY desktop blog editor even if the editor can't support custom fields!

Automattic

daniloercoli

Jorge Bernal

Make XML-RPC work if you rename the file. Some hosts block access to xmlrpc.php file making it impossible to use

Rename XMLRPC

Make XML-RPC work if you rename the file. Some hosts block access to xmlrpc.php file making it impossible to use

Text to Custom Field vs Rename XMLRPC