Two from Beyonce. The first is when George Michael joined her on stage in the O2 in London during her world tour to sing her song If I Was A Boy. The audio isn't the best but how amazing for the London audience to see these superstars sing together.
The second is when Beyonce, Pink, Britney (and Enrique too) made a soft drinks commercial in ancient Rome. It was pre-Spartacus thankfully.