Wall of Force
Wall of force is a great utility spell, and one that I think gets overlooked by novice spellcasters.
I use it to:
1. seal off escape routes to prevent an enemy from chasing my group as we retreat.
2. to keep potentially deadly foes at bay.
3. To divide enemy groups so that my group only has to deal with half of an enemy force at one time.
Recently, my party was in the underdark, chasing a necromancer who has become quite the antagonist. He had been aiding the vampire Gulthias in his efforts to create an army of undead to conquer a portion of the Flaness and help bring back the worship of Ashardalan. (See Heart of NightFang Spire) We defeated Gulthias, but Quinn the necromancer was able to get away. We gave chase and found an ancient drow temple that had been ruined centuries earlier, and that was now being used by beholders. After a few weeks in game time, we defeated the beholders and continued our search for Quinn. We finally found him in the bazaar of a Desmodu city. Quinn had now recruited some friends; a cleric, wizard, rogue and fighter. We could not attack this group openly, without incurring the wrath of local law enforcement, so we had to bide our time. I know, through experience, that NPC characters are usually the most difficult enemies to defeat, because they can usually match their group's strengths to yours, and sometimes they have the edge. In this case, they had 2 clerics and a wizard, all of whom could cast insta-death spells. It was obvious that my group was at a disadvantage. Tactics, and a Wall of Force, carried the day though. When we finally had the chance to attack the bad guys, they were already buffed up. We did not have the chance to prepare. (I won't explain why, because that would mean providing spoilers for Deep Horizon), but we definately started with a disadvantage. Fortunately, our rogue and my wizard had high intiatives. The wizard started with a mass haste, and he followed that with a wall of force. I was able to position the wall of force in such a way as to separate both clerics from he rest of the group. The only way those clerics were going to be able to bypass the wall of force was through Ethereal Jaunt, or through word of recall (which would have taken them completely out of combat) Fortunately for us, neither Cleric had ethereal jaunt.
The enemy wizard could have used disintegrate to bring down the wall of force, but he never got the chance. The rogue, who is actually a Temple Raider, had cast invisibility on himself. He then used his wand of silence on his own buckler and all he did after that was ready actions to follow the enemy wizard around, so that the enemy wizard was never able to cast spells. (Our DM learned from this tactic though, so in the future, we fully expect our enemies to have a few silenced spells.) This effectively took the wizard out of combat as well, and my group only had to deal with the enemy fighter and rogue. It only took a total of 4 rounds to defeat the enemy, and my group suffered almost no damage.
Without the wall of force though, it is highly likely that we would have lossed, considering the potential of the enemy clerics to cast harm, followed by melee attacks from the fighter and rogue. In addition, the Slay Living Spell would have meant certain doom for our wizard and rogue, and if the rogue had been killed, the wizard would have been able to get off some nasty spells as well.
"Divide and conquer" is a good guideline to follow when given the opportunity, and wall of force lets you do just that.