NPC formations

In real life, it's not just humans that use formations, in combat or otherwise. As usual, the real world has countless models which developers could adapt to gameplay, for AI behavior or otherwise.

Arrow formation
Birds often flock in an arrow formation, allowing all but the leader to expend less energy by flying in another's draft. Seagulls will stand in a similar formation on the ground, allowing most to rest easier since they don't have to struggle to remain balanced.

In a game, NPCs are dealing with real physics, so the drafting doesn't seem to have much application. But such formation may aid group casting and such. You might see two casters standing behind another, channeling their energy into him and thereby allowing him to cast spells beyond his level or collaborative-only spells.

And of course, aesthetically, it would be cool to see a herd of anything in a game, moving and shifting as a flock of birds or school of fish would.

Follow the leader
I once saw a video of a herd of water buffalo set upon by lions. The lions hid in the grass until they were able to sneak up and capture a calf. All the buffalo ran, except for the big leader. He stayed and tried to get the lions away from the calf. Eventually, the leader's courage inspired the others to come back and join the fight, at which point the lions didn't even have a chance.

That would be a cool scenario to see in a game. Imagine if your presence startled all but the leader. The others run away, but remain close enough to witness the fight. If you don't kill the leader quickly enough, the others will find their courage and enter the fight. They might be enough to overwhelm you, or they might simply stack the odds a bit. If you do kill the leader quickly, the others will really run away and not look back.

In general, it would be nice to see NPCs form up behind their leaders. They might try to remain close to the leader during combat, to retain his leadership group-buffs. There are probably many other interesting ways to tinker with the leader-follower relationship.

Ring formation
In many species, adults will form a protective circle around their young.

NPCs might form a protective ring around any valuable object. It might be an important character, a quest item, or just treasure. When you kill one, the circle becomes smaller and closes. A thief-type class might have a skill to rush through the gap (if he times it right), to sneak in a backstab or grab something on the inside, before leaping/tumbling over an NPC on the far side to escape before the NPCs turn.

Back-to-back
I'm not sure where all lovebugs live, but the South sees them all over the place at certain times of year. During the mating season, they're usually seen crawling and flying around while attached to one another at the abdomen.

Some NPCs might fight back-to-back if surrounded in a skirmish or an actual battle (I hope to see full army combat soon in MMOs). Some creatures may even attach as lovebugs do. The attachment might offer them buffs (kill one and the other loses its buff) or even allow them to share one healthpool (meaning you can no longer fight them one at a time or rely on reducing the odds halfway through the fight).

Lurkers
Archerfish lurk just beneath the water's surface and spit water out at unsuspecting bugs on nearby branches to topple them into the water, where the fish can eat them.

Imagine your group walking across and bridge or ledge when a creature attacks you (and only you) with some sort of missile, knocking you over the edge and down where it can get at you. Now you're stuck in the water or mud, separated from your party, having to fight this thing ambusher alone. Maybe it's something you can take alone, or maybe you call for your groupmates to jump down and help (maybe your group's distance fighters can help from where they're at, or maybe this creature has an extraordinary range and your group will all need to jump down).