Several species of spiders construct conspicuous and elaborate 'decorative' features in their webs. The features are termed 'stabilimenta' - incorrectly, since they are now known that they are not related to structural stability.
The practice is thought to have evolved via (at least) nine different evolutionary routes.
Many different ideas have been put forward to try to explain their seemingly enigmatic purpose - given that the decoration might alert some prey to the presence of the web. Some recent research has suggested that the decorations might actually reduce the number of captured insects (see: Behavioral Ecology, Volume 10, Issue 4, Pages 372–376) - though it might also prevent larger animals (e.g. birds) from accidentally destroying the web.
Further info (with photos) Wikipedia