The Common Spreadwing (Lestes sponsa) 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.
In The Netherlands, there are five different Spreadwings species (including the Willow Emerald Damselfly (Chalcolestes virides) that was formerly placed within the genus Lestes) that look quite similar.
The Common Spreadwing (Lestes sponsa) can be distinguised from the Small Spreadwing (Lestes virens) and the Migrant Spreadwing (Lestes barbarus) by looking at the back of the eyes (head): with the latter two the top half of the back is dark and the bottom half is yellow whereas with the Common Spreadwing it is all dark.
In addition, adult Common Spreadwings can be distinguished from the other Lestes species by their entirely dark pterostigma (wingspots).
Adult males show blue both at the end of their bodies (S9 and S10) and at the base (S1 and S2). 
Only the Emerald Spreadwing (Lestes dryas) shows this as well.
And finally, a very good way to distinguish the Small Spreadwing from the Common Spreadwing (especially in females) is to look at the side of the first segment of the abdomen (S1). 
The Small Spreadwing shows here a small dark spot (in Dutch: the "Neijtsvlek") whereas the Common Spreadwing doesn't.

1st photo: Oostvaardersplassen, The Netherlands, 4 August 2011
2nd photo: Botanical Garden, Utrecht, The Netherlands, 7 July 2011