Free download of Hi-Res HDRI Maps for rendering
There are lots of different ways to add an environment map to a scene to create reflections on mirrored surfaces or to add lights. To do that we have the HDRI maps which works really well with Cycles. But, to use HDRI maps you have to produce them, buy HDRI maps or use free libraries you found on the web. For quite some time I have here a link to three collections of free HDRI maps that I often use on my projects and classes. They are all hosted at DeviantArt and were created by the same artist called smashmethod.
And here are the links to the collections of HDRI maps: