PlantFiles: Marsh Violet, Bog Violet
Viola palustris

The marsh violet is very hardy, but covers a wide range of latitudes - from Scandinavia down to Spain, eg., so it can take warm climates as well.
It grows in upland bogs (full sun) and wet woodland (no sun) - so as you might guess, it prefers acid soils. I do not know, however, how much it might complain about alkaline sites (such as fens and limestone pond margins).
The kidney shaped leaves are fairly distinctive, but whether they are a useful differentiator depends on where you are - ie. what other species there are as candidates. In the UK, for instance, there is only one violet with kidney-shaped leaves - this one.
It will grow in moss and wet grass, and stands competition surprisingly well. It loses its leaves in winter, and spreads by runners as well as seed. The seeds are black.
The pale mauve flowers are small and unscented, but down at their own scale (amongst the moss) they re very dainty.
There is a subspecies (ssp. juressi) but it is fairly similar to the type.