I personally use and prefer VisualHeal (
http://ui.worldofwar.net/ui.php?id=3749) - cleanly implemented, works quite well, and provides both heals-on-target (for healers) and incoming-heals (for the rest of the raid) - for the former, it'll show any heals scheduled to land before yours, then your own heal, in a 3-bar stacked format (current HP, total heal before yours, your heal). For the latter, it shows current hp, next incoming heal, and then the sum of all other incoming heals.
(Of course, for predictive-heal display, the other members of your group must also be running VisualHeal.)