Dog's mercury (Mercurialis perennis) is not a lot to look at, indeed it is an insignificant plant than can look as though it has no flower at all. Close up, however, it has spikes of cream/green flowers in March and April. It is also a plant that has separate male and female forms. It does, to be honest, look a dull, rather boring plant.
It is, however, quite a significant flower as an indicator of primary (or long standing) woodland. It needs shade to thrive and so woodland is its preferred habitat and it spreads mainly by underground rhizomes and so forms large patches wherever it occurs. As it spreads slowly the larger the patch the older the woodland it is in is likely to be. Where you find it in any quantity look for other woodland species like wood anemone and wood sorrel. If you find dogs mercury outside of woodland then it normally means that a woodland once stood there but has been felled.
It is poisonous and should certainly not be eaten as it can cause all manner of problems including liver failure! It contains some harmful chemicals although I am not sure mercury is one of the ingredients! It could well be where the mercury in its name comes from though but what is more intriguing is why it is dog's mercury. Wikipedia suggests that it is because dog can mean false or bad however a website http://www.woodlands.co.uk/blog/flora-and-fauna/dogs-mercury has a number of comments describing how peoples' dogs find the plant irresistible although it usually makes them sick after eating it. Even though dogs are not normally considered vegetarian could it be that they find the scent and taste of the plant so attractive that they eat it even though it is poisonous and it makes them sick?