Post selector as Custom Field

The excellent My Metabox class by bainternet does exactly what you are looking for. Add a post dropdown list with

$my_meta->addPosts('posts_field_id',array('post_type' => 'post'),array('name'=> 'My Posts '));

And it will create a meta box with posts dropdown (you can change this to a checkbox list if you want to be able to select more than one post)

My Metabox class can be customized to your liking with little effort.

Edit: It’s My Metabox Class not Tax Meta Class, which lets you create taxonomy meta data in the same way.