Originally Posted by Mags

My all-purpose healing macro (a variation of some of the one suggested up thread):

#showtooltip Greater Heal(Rank 7)
/stopcasting
/cast [target=player, modifier:alt]Greater Heal(Rank 7);[target=mouseover,help][help][target=targettarget,help][target=player]Greater Heal(Rank 7)

If you press down Alt, it self-heals.

Otherwise it chooses a target depending on which condition is true:

Friendly mouseover target? Heal them.
Otherwise heal my target.
If my target is hostile, heal my target's target
If nothing is targeted, self-heal.

You have to be careful where your pointer is (its easy to heal a random mouseover target, rather than the person targeted that you want to heal) but otherwise its a very versatile macro.