Mozart composed the motet Ave Verum Corpus less than six months before his death in 1791. It is a setting of a 14th century hymn in Latin; the words can be approximately translated into English as:
Hail true body
Born of the Virgin Mary
Who truly suffered, sacrificed
On the cross for the sake of mankind;
Whose pierced side
Flowed with water and blood;
May it be for us a foretaste (of heaven)
In the trial of death.
Despite being only 46 bars long, the piece is viewed by many as one of Mozart's most perfect works. It was originally scored for a four-part choir, strings and organ, but many recordings use a string orchestra of a size that rather overpowers the voices. The most enjoyable performances - from a singer's perspective at least - are those that use a chamber-size string ensemble, such as this one by the Concordia Choir of Moorhead, Minnesota. Many conductors also take the piece at a very slow tempo, and manage to make the 46 bars feel indeed like a foretaste of eternity; a notable exception is this performance by the Moscow State Chamber Choir and Orchestra, which plays with the tempo in a way that just seems to suspend time altogether. It's worth taking the trouble to turn up the volume.
A capella performances are a little harder to find, and in general it seems that the larger the choir, the slower the tempo and the blander the performance. Here is a startlingly engaging turn by an anonymous and very casually dressed male voice quartet.
Ave Verum Corpus is also widely sung as an aria, usually by a soprano or tenor with organ or string accompaniment. This recording by a Peruvian trio is less conventional (soprano, guitar and clarinet) but is nonetheless outstanding.
The strength of the tune has made it a popular instrumental piece too, played on a wide variety of instruments. Here are three good ones, by a string quartet, a violin with piano accompaniment, and as an arrangement for classical guitar. Special mention should also go to an arrangement for solo piano by the 19th century German composer Franz Liszt.
For a surprise performance (this time literally), here is the Italian choir Coro Incontrocanto singing Ave Verum Corpus as a flash mob in a shopping gallery in Turin, and turning in a performance that ranks with the best.