What's the most advanced tech you can emulate with this?

[SangoProduction]

A {wand} of permanent Unseen Servant.
It is in all ways the same as a normal Unseen Servant, except that it has no duration. Also, to be able to get it to do anything, you must spend an hour "programming" it to do a very specific range of related tasks (like pressing buttons on a pad, or flipping levers). After said programming, it must can't be reprogrammed until it's destroyed.

What is the most advanced, and useful tech that you can come up with which uses this as a central part?

I was thinking a remote control would be a funny, if wasteful, use of it. You just speak which buttons you want it to press on the televi..I mean crystal ball, and it does so.

I could also see a rather weak engine which uses this is (at least) a back up propulsion, or to "spin up" the engine, like old-time planes needed.

[Fizban]

Depends on how many you have. As a shapeless force that is not even a creature, there's no limit to how how many you can pile onto how small of a point (unless PF changes this of course). Massed Unseen Servants aren't the highest energy spell, but they are the absolute easiest to turn into industrial machinery. Pretty sure they're cleanroom compliant too.

[Eldan]

Just one unseen servant, or several? Because if you can get an array of them, you could build a computer. Just program each servant to act as a logic gate processing a simple input into an output.

[SangoProduction]

Just one unseen servant, or several? Because if you can get an array of them, you could build a computer. Just program each servant to act as a logic gate processing a simple input into an output.

I originally had in the draft "The cost increases exponentially with each additional servant." But decided to do away with it, since, well, this board likes to know exactly what is meant by what is said, rather than the idea behind it, and it wasn't a huge deal. The idea was "What's the most inventive thing you could do with as few unseen servants as possible."

[unseenmage]

Combined with Planar Ring Gates (PlH) strewn throughout many extra dome signal storage spaces you could have them fetch and carry for you from far off exotic locales.

Anything in any connected space brought to your Handy Haversack at a moment's notice.

[Fizban]

If you want to know the most that can be done with a small number of programmed Unseen Servants, you'll first need to properly define the limits of that programming. The computer example, for example, can be imagined as each unseen servant being used as a single on/off value, but that's not very useful. But you're allowed to "program one for an hour," which is rather more significant, and compounds as you chain them together and invent programming. Once again, as shapeless forces there's no reason you can't miniaturize whatever setup you're using to facilitate sending signals between them.

With some number (probably a dozen or two would do the job) you can pretty much skip past the need to invent engines to power machine tools, which are the bar for a lot of things. If the permanent USs are prohibitively expensive then you won't have large scale mass production, nor can you use them to skip over power sources for tranpsortation of goods- but DnD already has solutions for those and even a single workshop that can produce standardized goods is pretty dang useful. Custom infinite Fabricate items obviously win this, but if those don't exist, Unseen Servant based machining is a pretty big step in fabrication.

The shapeless force aspect leaves some more question as to what it can do. Can it move water without a bucket? Air? Can it move vertically or just "hover" across the ground? With a few props there are all sorts of automated systems you could power with a permanent "programmed" US, especially considering its autonomous movement. Heating/cooling, alarms, doors, filing and organizing, handling hazardous materials (anything that doesn't require it to take damage from area effects). The only limit is what you define as a "very specific range of related tasks". How is this program entered, spoken commands? A person can read a lot of words in an hour.