My character is a cleric necromancer looking for utility-type undead. I'd like to find a low-HD undead critter to Command who would be able to act as an Unseen servant (pick up small objects, trigger traps, etc). Ideally it wouldn't be much bigger than 1-2HD including any turn resistance.
Pathfinder to the rescue!
In Pathfinder you can find the 3.5e-compatible CR2 Poltergeist. It's an incorporeal invisible skeletal ghost with telekinesis and scare powers. It comes originally from Tome of Horrors for 3.5 so if you want a "pure" 3.5e version it's to be found there.
Good news, by RAW you have a couple of options.
First option: A wand which casts the spell Unseen Servant (Wiz 1). Crafting a wand that does this with 5 charges/day = 1,800 GP. That's relatively cheap for 5 casts that recharge daily.
Second Option: Get a wand or some scrolls that allow you to cast Invisibility on a target. Then use Animate Dead, and make the creature invisible.
Animate Dead, PHB pg. 198 (text only)
This spell turns the bones or bodies of dead creatures into undead skeletons or zombies that follow your spoken commands.
The undead can follow you, or they can remain in an area and attack any creature (or just a specific kind of creature) entering the place. They remain animated until they are destroyed. (A destroyed skeleton or zombie can't be animated again.)
Regardless of the type of undead you create with this spell, you can’t create more HD of undead than twice your caster level with a single casting of animate dead. (The desecrate spell doubles this limit)
The undead you create remain under your control indefinitely. No matter how many times you use this spell, however, you can control only 4 HD worth of undead creatures per caster level. If you exceed this number, all the newly created creatures fall under your control, and any excess undead from previous castings become uncontrolled. (You choose which creatures are released.) If you are a cleric, any undead you might command by virtue of your power to command or rebuke undead do not count toward the limit.
Skeletons: A skeleton can be created only from a mostly intact corpse or skeleton. The corpse must have bones. If a skeleton is made from a corpse, the flesh falls off the bones.
Zombies: A zombie can be created only from a mostly intact corpse. The corpse must be that of a creature with a true anatomy.
Third option: Take one level in Wizard, then you can just have Unseen Servant as a spell without any headaches with trying to adjust it to fit.
Non-RAW option: Discuss with your DM the requirements for penning an actual spell which performs the Unseen Servant spell from the Wizard spell list as a Cleric spell. Your DM may allow this if it's in keeping with your character. I would recommend coming up with a convincing spell that's in keeping with both Animate Dead and Unseen Servant.
Animate Dead: Requires 25GP in black onyx gem per hit die of the undead inserted into the eye or mouth of the creature, and is a combat creature.
Unseen Servant: Requires a bit of string and some wood, but isn't a combat creature.
Combining these into one spell, I would rule the following:
Animate Poltergeist: Requires 25GP in small gems inserted into the creatures eye or mouth, and is an invisible, non-combat creature that follows you as long as you like / until it's destroyed / dismissed.
This way, your spell meets the requirements of Animate Dead for cost, while incorporating the requirement of a corpse to initiate, as well as the utility you are looking for in Unseen Servant. As a bonus, making it a spell means you can only use it so many times per day which would prevent you having a permanently free trap trigger.