"Viili (Longmilk, Finnish viili, Swedish fil) is a type of yoghurt (a mesophilic fermented milk) that originated in the Nordic countries. It has a ropy, gelatinous consistency and a sour taste resulting from lactic acid." (Wikipedia)
I don't buy often longmilk, but when I do, I use it as a starter for a whole batch of longmilk. This homemade longmilk tastes nicer (less sour) than the industrial ones and is very easy to make: Just mix 1-2 spoonfuls of longmilk with about a cup of milk, cover it and leave on the table for about 24 hours. When ready, keep refridgerated.
Enjoy with berries, cinnamon, sugar, or like I do: just on its own.