Who are the little people who clean in exchange for a saucer of milk?
Terry Pratchett has referenced them in several works. I think other authors have too. I've googled, to no real avail.
They first have to "steal" the instructions from manuals, the back of packets, etc. before they can do any of their work, which mystifies the Housedwellers they've attached themselves to. Possibly they 'emigrated' with an Irish or Scottish family, but given how little they have to do now they've decided to expand to the house next door.
In English they are normally known as Brownies. I could write a bunch about them but wikipedia covers it pretty well.
A brownie/brounie or urisk (Lowland Scots) or brùnaidh, ùruisg, or gruagach (Scottish Gaelic) is a legendary creature popular in folklore around Scotland and England (especially the north, though more commonly hobs have this role). It is the Scottish and Northern English counterpart of the Scandinavian tomte, the Slavic domovoi and the German Heinzelmännchen.
In 1703, John Brand wrote in his description of Shetland (which he called "Zetland") that:
Not above forty or fifty years ago, every family had a brownie, or evil spirit, so called, which served them, to which they gave a sacrifice for his service; as when they churned their milk, they took a part thereof, and sprinkled every corner of the house with it, for Brownie’s use; likewise, when they brewed, they had a stone which they called "Brownie’s stane", wherein there was a little hole into which they poured some wort for a sacrifice to Brownie. They also had some stacks of corn, which they called Brownie’s Stacks, which, though they were not bound with straw ropes, or in any way fenced as other stacks used to be, yet the greatest storm of wind was not able to blow away straw off them.
From a Google books preview of The Folklore of Discworld, in the chapter The Elves (the Wikipedia page for Nisse redirects to Tomte so I just bolded it):
There was even one type, the house-elves, whom humans welcomed. The English called them hobs, pixies or pucks, the Scots brownies, the Scandinavians nisses and tomtes. These would actually live in a farm and bring it luck; they would help with harvesting, tend the animals, even do the housework, in exchange for an occasional bowl of milk or porridge - provided nobody spied on them or laughed at them. Russian country folk said there were several on each farm; the most important one lived behind the stove, others guarded the barn, the bath-house, the henhouse, and so on. On the Discworld, only the Wee Free Men have ever done such a favour for humans, and then only once, in the very special circumstances created by their bond with Tiffany Aching. Their reward was Special Sheep Liniment, which smells suspiciously like whisky.
Note that they apparently weren't actually little in Discworld:
Another sign that people in Europe were forgetting the true nature of elves, and no longer took them seriously, is that they so often thought of them as small. The Little People, the Wee Folk. Some people said they were about the size of a rabbit; others, that of a six-year-old child....