Example of pulling posts from another site in a MultiSite install

<?php
global $wpdb; // Globalize the $wpdb object so you can use it

/* Set this next variable to whatever the ID is of the sub-site
you want to pull post(s) from */
$sub_blog_id = 2;

/* This next line of code will cause most WordPress functions to pull
information from the site with an ID matching $sub_blog_id instead of
pulling it from the current site */
$oldblog = $wpdb->set_blog_id( $sub_blog_id );

/* Use the get_posts() function - adding whatever parameters you
desire - to pull information about the post(s) you want to pull
from the sub-site */
$sub_posts = get_posts();

/* Loop through the posts you retrieved */
foreach( $sub_posts as $sub_post ) {
  // Do stuff here to output your desired information
}

/* Tell the $wpdb object to go back to using the current site */
$wpdb->set_blog_id( $oldblog );
?>