This is a handy little script that adds the same amount of health and SP to the heros' current HP and SP as the max HP and SP. For example, as Tomtaru said, the hero enters battle with full health 10/10. In battle he is injured to 6/10 HP. When he defeats the monster(s), he gets enough exp to level up. Not only does the hero's max HP rise by 10, but so does his current HP. So at the end of the battle, the hero's health is now 6/10 + 10/10 = 16/20.
I thought it might be a good idea to post it in this forum so that more people could see it and benefit

Open the Game_Actor script and find the exp=(exp) attribute (just use ctrl+f to search for it). Replace the code of the method with this:
CODE
# exp=(exp) attribute that allows level up healing
# modified by: Jako Drako
# Give credit!
def exp=(exp)
temp_level = @level
@exp = [[exp, 9999999].min, 0].max
while @exp >= @exp_list[@level+1] and @exp_list[@level+1] > 0
@level += 1
for j in $data_classes[@class_id].learnings
if j.level == @level
learn_skill(j.skill_id)
end
end
end
while @exp < @exp_list[@level]
@level -= 1
end
if temp_level != @level
actor = $data_actors[@actor_id]
level_difference = @level - temp_level
@hp += actor.parameters[0, @level] - actor.parameters[0, @level - level_difference]
@sp += actor.parameters[1, @level] - actor.parameters[1, @level - level_difference]
if @hp > maxhp
@hp = maxhp
end
if @sp > maxsp
@sp = maxsp
end
end
end