WordPress MU

Hi eveyone..
Im trying to add a script that will display a block of popular tags from Site-wide in the post writing page...
or add a word complition to tags from the whole tags exists in the site not only the blog.
my quastion is how to collect the whole tags and how they are stored in the database...

If there is any plugin that to do that job, ill love to use it, im not assiting on inventing the wheel.

http://wordpress.org/extend/plugins/wordpress-mu-sitewide-tags/

Hi andrea thx for the help...
but i have another quastion...

in the post writing page there is a block of tag-cloud that displays the popular tags from the blog...

after installing the plugin you gave its really easy to change the blog id to the tags blog...
but i cant find the place of the tag-cloud box code...
could you point me to that place?
i've searched the whole files for this tag-cloud...