If you need just some random ideas, then:
1. Not actually a summon, but a ward. The spell creates a rod/ward/obelisk/totem that links itself to another ward of it's own kind. And whenever an enemy unit comes in between two or even few of these wards - well, something happens

. The unit is constantly struck by fire, lightning, or is having it's mana drained. Maybe you can turn it into a trap that stuns all units in between two wards (the wards are destroyed by this).
2. A spell that: has already been suggested by
Cleavinghammer; will probably be used by me; could be already seen in other maps; is still a nice idea.
Flesh Golem/Wraith Spawn/Any other epic undead name - A spell that "consumes" all dead bodies in the targeted area and summons a powerful unit. Power and strength of the sumoned unit will depend on the amount of consumed dead bodies. The unit should also have some abilities
other than passives. Keep those corpse carts and meat wagons with their infinite amount of dead ghouls in mind

. Adding a power limit should balance the things out.
3. A summoning spell that initially deals damage as a spell and then spawns a unit. Pretty much like the Infernal Golem spell in Warcraft. The summoned unit itself is quite tanky and devastating, and the fact that it's falling from the sky right on the heads of your enemies allows you to put a tank right into the action, or even behind the enemy army's front line. You can come up with something interesting like a huge, rolling boulder which, upon collision with building/cliff/etc, turns into a pumped-up rock golem (as an alternative to infernal golem). Or you can replace the rock golem and boudler with, let's say, a raging bear that rushes forward as it spawns.
I'd also like to remind you that any summon which has non-passive abilites or suits a certain role or purpose is always a good summon. Esepcially if this summon compelemnts the hero who's summoned it. (Careful not to turn this hero into an OP jack of all trades)
A summon can also provide some neat utilities like scouting, healing, Control Effects (stun, slow, root, etc.) or anti-spellcaster/target-damage abilities.