The Willow Emerald Damselfly (Chalcolestes viridis, formerly Lestes viridis) is a species of damselfly in the family Lestidae, the Spreadwings.
Spreadwings hold their wings at about 45 degrees to the body when resting. 
This distinguishes them from most other species of damselflies which hold the wings along, and parallel to, the body when at rest.
The Willow Emerald Damselfly can be distinguised from the other Spreadwings (Lestes species) by its yellow/brownish pterostigma (wingspot) and by having a little green marking in the shape of "thorn" on the side of its thorax.

1st photo: Zanderij Crailo, Hilversum, The Netherlands, 29 August 2013
2nd photo: Bussum, The Netherlands, 15 September 2016
3rd photo: 's Graveland, The Netherlands, 28 August 2012
4th photo: Grotelse Heide, Bakel, The Netherlands, 17 July 2018
5th photo: Landgoed Oostbroek, De Bilt, The Netherlands, 4 September 2018