16th level Cleric power out of Divine Power.
Encounter power: Each target (i.e. ally) gains temporary hit points equal to his or her healing surge value. Temporary hit points do not reset until a rest. So, use the power after each rest.
If the 16th level PCs have 100 average hit points (a bit low) and there are 5 PCs including the Cleric, then that's 100 free temporary hit points per encounter. 5 Encounters = 500 extra temporary hit points. At level 30, that could easily be 170 or more temporary hit points per encounter or over 1000 temporary hit points if the day has 6 encounters.
The same 16th level Cure Critical Wounds would be ~75 hit points per day at level 16, not 500.