1) Intelligent constructs of various sorts: I'm fond of the sort of 'talking door' trope used in Labyrinth, for example. Often these are abandoned by their original creators, and are no more loyal (and no less) to the current inhabitants than the PC's.
2) Non-hostile monsters: Not every monster is spoiling for a fight with a well armed party of humanoids, either out of fear, laziness, boredom, or economic calculation. Many a monster would rather haggle for a bribe to let the party pass, and some - sometimes surprising ones - are simply in the mood to talk. It's fun to have the party run across a talkative troll, ogre, giant, or dragon from time to time especially when the creature reveals surprising depth (the troll is master violinist, for example). It can also include monsters that have evil motives, but want to express that evil in a more subtle manner than immediate assault and battery. For example, a monster that wants to seduce or corrupt the party. These often present themselves as something else on this list entirely. For example, the intelligent construct might actually be a ghost animating an object, which will use a magic jar attack at a later point. Or the prisoner might well be a doppleganger in disguise. The good pixie might turn out to be an evil witch, etc.
3) Freeboarders, hangers-on, and guests: In my game, these are often fairies, but they can include any humanoids that are tolerated by the more power dungeon denizens but which are not in fact very loyal to each other. For example, an evil drawf smith might work for a tribe of goblins. Since the smith has no real loyalty to the goblins, he may be perfectly content to let the party go on its way if they let him go on his. A travelling Derro sage may be the guest of a group of Gnolls, but have little enough affection for his hosts. A sprite may live in little used storeroom or forgotten chamber, and be different than a rat only in that you can more easily role-play with it. A kobold may serve as the pet or syncophant of a much more powerful creature.
4) Prisoners: If the dungeon dwellers are intelligent, they may have prisoners. As Gygax showed, these prisoners can have motives of their own, making the decision to free them not necessarily a given.
5) Fellow Adventurers: I would caution against this one being used too often and without a lot of forethought. Having other adventurers in a dungeon can make the place crowded, and it tends to violate the general rule of not having more than one NPC on stage at for a long period. If their motives are similar to the PC's, you risk making the NPC's the center of attention. If the motives are dissimilar, this probably ends up as a rather hard fight that may distract from the challenges of the dungeon itself. If the NPC's are victims in waiting, the PC's may be annoyed to have to play nurse maid. If you do decide to go this route, make sure you plan for the encounter to be the focus of the dungeon.
6) Friendly Neighbors: By this I mean, friendly to the PC's but not necessarily to the dungeon inhabitants. If the dungeon is large enough, it may be divided amongst several factions, some of which may be friendly to the PC's. Indeed, while this usually means normally 'good' creatures because otherwise the PC's will tend to shoot first and ask questions later, it may well mean simply a faction that thinks it can use the PC's to its advantage. Like a tribe of goblins that wants to wipe out a rival. It can also mean fairly bizarre things, like the spirits of some good folk who would like to assist the PC's in avenging their deaths at the hands of the current dungeon inhabitants.
7) Polymorphed Creatures: I'm fond of fairie tale tropes, so that unusually friendly monster might turn out to be a prince under a curse, or a princess whose been polymorphed into something and forgotten that she is a princess. Yet another reason for not attacking everything ugly you encounter.