what does SHOW /*!50000 GLOBAL */ STATUS do ? What is the difference between 

SHOW /*!50000 GLOBAL */ STATUS 


and 

SHOW GLOBAL STATUS


I saw that in few scripts and tuning presentations the first command is being used. Is there a difference, If there is, what does the 50000 signify ? 

Also, this seems to be a good script : https://raw.github.com/rackerhacker/MySQLTuner-perl/master/mysqltuner.pl 
Are there any similar scripts that list important stats and display recommendations
The "50000" refers to a MySQL version -- it means that only version 5.0.0 and above should pay attention to that command.  5.0.0+ will see SHOW GLOBAL STATUS, while earlier versions will see just SHOW STATUS.

Example:

mysql&gt; /*!50511 select 1 */;
+---+
| 1 |
+---+
| 1 |
+---+
1 row in set (0.00 sec)

mysql&gt; /*!50512 select 1 */;
Query OK, 0 rows affected (0.00 sec)


(I have MySQL 5.5.11 -- i.e. 5.05.11).