WordPress.org

1. Install easily with the WordPress plugin control panel or manually download the plugin and upload the extracted folder to the /wp-content/plugins/ directory
2. Activate the plugin through the 'Plugins' menu in WordPress
3. Modify the appropriate post queries with the orderby parameter set to post__in

Example

query_posts( array( 'post__in' => array(20,10,106),  'orderby' => 'post__in' ) );

That will retrieve posts with IDs 20, 10, and 106 in that order!