This is the Grand Prismatic Spring in Yellowstone National Park - the largest hot spring in North America and the third largest in the world. The incredible colours are produced by pigmented bacteria that grow in microbial mats around the edges of the mineral-rich water. In summer, the mats are usually orange and red, and in winter they're are usually dark green.
Image: 4lexandru (flickr.com/photos/4lexandru)