Popper has co-written songs with Trey Anastasio, Warren Haynes, Chris Barron, and Jonny Lang. He also frequently appears as a guest performer with musicians both famous and obscure, from a diverse variety of genres. He has performed with, among others, jam bands Spin Doctors, Dave Matthews Band, Phish and most recently, the Allman Brothers Band in 2009; bluesmen Eric Clapton and B.B. King; singer-songwriters Jason Mraz and John Mayer; saxophonist Karl Denson; San Francisco's Culann's Hounds; heavy metal band Metallica; rock trio, ZO2 and even with the Hungarian Ambassador to the United States, András Simonyi. He sat in with the Smashing Pumpkins on the second day of their acoustic 1997 Bridge School Benefit appearance, contributing harmonica for their song "Porcelina of the Vast Oceans"; Popper's solo garnered major applause from the audience. He also performed with the Grateful Dead at a tribute to Bill Graham in 1991. He was the harmonica player on Hanson's album "This Time Around" in 2000, for which he performed on "If Only" and "In The City." He also composed, wrote and performed the theme song for the ninth season of the sitcom "Roseanne".