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!

Max Cutler

This plugin brings XML-RPC API enhancements from the WordPress 3.4 release to previous versions 
of WordPress (3.3 and earlier). It also adds new user management methods.
It is derived from Prasath Nadarajah’s GSoC ’11 project to expand WordPress’ web services, 
although the exposed API methods are not compatible (different parameter names/types/orders).
<h4>Methods</h4>
New Methods:
<ul>
<li>wp.newUser – create a new user</li>
<li>wp.editUser – edit user information</li>
<li>wp.deleteUser – delete a specfic user</li>
</ul>
3.5 Methods for pre-3.5 sites:
<ul>
<li>wp.getUser – get information about a specific user</li>
<li>wp.getUsers – retrieve a list of users</li>
<li>wp.getProfile – retrieve information about the requesting user</li>
<li>wp.editProfile – edit the profile of the requesting user</li>
<li>wp.getRevisions – retrieve revisions for a specific post</li>
<li>wp.restoreRevision – restore a post revision</li>
</ul>
3.4 Methods for pre-3.4 sites:
<ul>
<li>wp.newPost – create a new post (of any post type)</li>
<li>wp.editPost – edit a post (of any post type)</li>
<li>wp.deletePost – delete a post (of any post type)</li>
<li>wp.getPost – get a specific post (of any post type)</li>
<li>wp.getPosts – get a list of posts</li>
<li>wp.getPostType – get information about a specific post type</li>
<li>wp.getPostTypes – get a list of registered post types</li>
<li>wp.getTaxonomy – get information about a specific taxonomy</li>
<li>wp.getTaxonomies – get a list of registered taxonomies</li>
<li>wp.newTerm – create a new term in a taxonomy</li>
<li>wp.editTerm – edit a term in a taxonomy</li>
<li>wp.deleteTerm – delete a term in a taxonomy</li>
<li>wp.getTerm – get information about a specific term in a taxonomy</li>
<li>wp.getTerms – get a list of term associated with a taxonomy</li>
</ul>

XML-RPC Modernization

excelent

This plugin updates the WordPress XML-RPC API to leverage the latest features of WordPress and move beyond the historical Blogger/metaWeblog/MT APIs.

Text to Custom Field vs XML-RPC Modernization