Simple...:
CODE
SELECT count(members.*) as members,count(posts.*) as posts,count(topics.*) as topics,count(online.*) as online FROM members_table members,posts_table posts,topics_table topics,online_table online
replace members_table with your users table,posts_table with your forum posts table,topics_table with your topics(threads) table,online_table with your 'who's online' table.
PHP
<?
/* connect to db,etc */
$query = mysql_query("SELECT count(members.*) as members,count(posts.*) as posts,count(topics.*) as topics,count(online.*) as online FROM members_table members,posts_table posts,topics_table topics,online_table online");
$row = mysql_fetch_array($query);
foreach($row as $key => $val)
{
echo $key.' : '.$val.'<br/>';
}
?>