One of the high points of the Easter Vigil liturgy is the chanting of the Exultet, a hymn in praise of the newly lighted pascal candle, which is a very rich symbol of the risen Christ, with links to both Passover and the Exodus.  It is very ancient in the western church, originating during the 5th-7th centuries.  Traditionally, it is chanted by a deacon, as in the picture at left, though in many congregations without a deacon--or at least a deacon who can chant well--it is sung by a member of the choir. 

Some more information on the Exultet is available here.  I also include a video of a good English version.