Discord Unit Frames is one solution.
Another possibility would be Satrina Unit Frames (
http://www.wowinterface.com/download...fo.php?id=4164 ). They're rather unsightly in my opinion, but they do show just numbers instead of graphical bars.
I don't know of any addons that completely change the bars to text in the Blizzard default unit frames. It might be possible to use MobHealth3 (
http://www.wowinterface.com/download...php?s=&id=5224 ) with its Blizzard frames plugin ( linked on the download page ) and then use another addon like MoveAnything to hide the graphical bars. I'm not 100% sure you can hide them, though. You could always give it a try.