WordPress has a convenient function that can create a new post: wp_insert_post(). Suppose that you wanted to create a plugin to automatically create weekly roundups of your social media activity. You could gather your Delicious bookmarks, Twitter posts, etc. with SimpleXML, mash the data up into a coherent post, then publish the post.

The function syntax is along these lines:

global $user_ID;
$new_post = array(
'post_title' => 'My New Post',
'post_content' => 'Lorem ipsum dolor sit amet...',
'post_status' => 'publish',
'post_date' => date('Y-m-d H:i:s'),
'post_author' => $user_ID,
'post_type' => 'post',
'post_category' => array(0)
);
$post_id = wp_insert_post($new_post);

One thing I find particularly interesting is the post_type argument. You could change it to “page” to create a page instead, or you could combine wp_insert_post() with the (as of yet ill-documented) custom post type API to create new admin panels for your post types.

You can edit a post by passing a post ID in the appropriate argument, select multiple categories in the array, or do pretty much anything you can do from the Write screen. After creating the initial post, you can even use the ID that is returned to add custom fields. The full list of parameters for wp_insert_post() is available in the Codex.