I have 3 of them. The magnets in the ones I have are not very strong so I have to support them so they dont fall down if too much weight is put on them. They are like these.
http://www.jlaquatics.com/phpstore/s..._ID=dc-st28763
You can also make your own.
http://www.reefkeepingforums.com/index.php?topic=211.0