In what song do the Red Hot Chili Peppers pay tribute to Kurt Cobain?

"Tearjerker" was partly a tribute to Kurt Cobain and Anthony Kiedis' reaction to his death.

https://en.wikipedia.org/wiki/Tearjerker_(song)

The video, but not so much the song itself, for "Dani California" also makes a reference to Kurt Cobain and Nirvana.

http://www.contactmusic.com/new/xmlfeed.nsf/story/nirvana-fans-furious-about-chili-peppers-video_19_05_2006

The hit song "Californication" has a line that says "And Cobain can you hear the spheres singing songs off station to station"
RIP Kurt Cobain...you touched a generation and a world like no other man ever did or ever will...though your death was officially ruled a suicide, i know in my heart somebody killed you. Your music and legacy will live on forever.

" Californication " mentions Kurt Cobain.

Space may be the final frontier, but it's made in a Hollywood basement
And Cobain can you hear the spheres singing songs off station to station

http://www.lyrics007.com/Red%20Hot%20Chili%20Peppers%20Lyrics/Californication%20Lyrics.html