The basic is MarkMyTarget, which places a raid icon of my choice on whatever I am currently targeting. This is quite handy for AoE pulls, just in case the DPS actually cares what individual thing I am facing (they don't).
For full raid marking, I use MagicMarker. I have the hotkey set to Control, so if I hold down Control and mouseover targets, it marks them in a pre-specified marking order.
I can think of two fights in Naxx where I actually use MagicMarker - the rest of the time it's all MarkMyTarget.