Quartz can do that besides many other things castbar related, maybe you already got it installed. Its THE castingbar mod. :D
(You have to resize the castingbar to width 0 etc. and move the time to the buff)
Well, "Addon Ideas" would have been the proper forum, although the description unfortunately neglects to mention that it is the appropriate place for asking if there is an existing addon to do something. However, a forum described as "Authors, post your release ready addons here to get feedback or just to let everyone know about it." is obviously not the correct place. Moving this thread.
Also, Quartz does have a debuff display... but it is bar-based, not icon-based, and you cannot configure it to show only the icons. I don't know of any standalone addons that will show you target debuffs as icons. I know of several that do so for player buffs and debuffs, but as far as I know none of them support other units. ElkBuffBars, despite its name, can be configured to show only icons, but only in a vertical stack, which probably doesn't help you either.