Larloch's Minor Drain: Regardless of whether the spell has an effect on an enemy, the caster's maximum HP increases by 4, and the character is healed 4 points.

Polymorph effects: You can backstab while polymorphed. Doing so while an Iron Golem will allow you to do lots of damage, even enough to kill the SoA final boss. (I saw a speedrun do this with Yoshimo; on a casual playthrough, having one of the thief/mages do this is more practical.)

Sunfire: This spell makes you immune to fire for 3 seconds. Hence, with the right timing, you can use it to protect yourself from an enemy's fire attack, or you can have two characters cast the spell at the same time to avoid hurting each other. (Of course, if exactly one has her spell disrupted, it will hurt.) Also, if the spell goes off while you are casting something else (with Contingency, for example), your spell will be disrupted.

Magic Resistance: Sets Magic Resistance to a specific value, allowing you to use the spell offensively against enemies with high Magic Resistance. (I note that the IWD version adds to Magic Resistance, preventing this weird strategy from working.)

Also, in Icewind Dale:
Spiritual Hammer and Mordenkainen's Sword: These spells create melee weapons that have an unusually long range (I believe 40, where line of sight is about 26). Therefore, you can attack that Frost Salamander that's on the other side of the screen and get damaged by its aura.

(By the way, which version of Mordentainen's Sword do you prefer: The BG2 version (which is a summon) or the IWD version (which creates a magic weapon)?)

Mordenkainen's Sword should be a summon. From the description:

"If the sword goes beyond the spell range from the character, if it goes out of the character's sight, or if the character is not directing it, the sword returns to the character and hovers."