Previously had "Tooltip" but no longer supported.
Now have Tipsy. Easier to use.... type "/tipsy show" position the Tooltip where you want and right click the box to lock it....sorted.
http://wow.curse.com/downloads/wow-addons/details/tipsy.aspx
Not that I guess it makes much difference, but i'm using LitheTooltipDoctor to do exactly the same thing. Just as easy to use as well.