Force of Nature (FoN for short) is able to give +3 bonus HP to an ally monster of your choice for only 1G, making it the spell equivelant of Migospel.
FoN works very well on monsters with Taunt. Also very effective on Papyrus to keep him alive so he can clear out the entire board. FoN can also be used to provide HP to offensive monsters to help apply pressure towards the opponent.
Since Beta 52.1, it can also target the user themselves. This makes it function just like Innkeeper and is a very easy way to increase your own max HP as the only spell to do so (after Longevity was changed in the same update).