Before Katy Perry became famous in 2008, she was part of a number of projects that never really took off. Probably her biggest claim to fame before "I Kissed a Girl" was recording a song with nu-metal band P.O.D.
Perry appeared on the band's lead single "Goodybe For Now" off their sixth studio album Testify in 2006. At this point, P.O.D.'s popularity was dwindling, as was the whole nu-metal genre. Besides appearing in the music video for the song, she appeared with the band on late night promoting the single.
Here is the music video for "Goodbye For Now." Katy can be seen starting around the 3:00 mark: