The Tallis Canon
A canon is a melody that can be sung in parts. One voice starts singing and then a second voice starts the same song several beats later and it blends and sounds good. The most well known canon is probably Row Row Row Your Boat.
Lots of composers have written canons. Bach wrote fugues which are like canons but more complicated.
Thomas Tallis wrote a hymn in 1567 called "God Grant with Grace". It can be sung in canon and also became known as “Tallis’s Canon.” New words were added around 1674 by a different English composer, Thomas Ken.
The Tallis Canon is still sung today all over the world.
Here is the music to the Tallis Canon. It can be sung in unison, and from 2 to 8 voices or parts.
Download a printable version here (originally from Total Choir Resources).
Here are some performances of the Tallis Canon.