On what tour was Gary Numan invited as a special guest by the Nine Inch Nails?
Through the years, Reznor has invited many prominent musicians on stage with his band to perform material outside the usual range of Nine Inch Nails songs:
During Lollapalooza '91 Jane's Addiction members Dave Navarro and Eric Avery played guitars alongside Gibby Haynes and Ice-T for the last song of the band's set, "Head Like a Hole".
In early 1995, Adam Ant and Marco Pirroni joined Nine Inch Nails on stage to perform "Physical" and other Adam and the Ants songs.
During the Dissonance tour, Nine Inch Nails co-headlined with David Bowie, whose own contemporary tour was called the Outside Tour.  Throughout the tour, Nine Inch Nails would perform first and segue into Bowie's band.  The two bands would play a mixture of Nine Inch Nails and David Bowie songs.  Nine Inch Nails would eventually leave the stage and Bowie and his band would continue with their own set.
Marilyn Manson appeared on stage at during a concert at Madison Square Garden in 2000 to sing "Starfuckers, Inc." and "The Beautiful People".  This performance is featured as an Easter Egg in the And All That Could Have Been DVD.
During the Live: With Teeth amphitheater tour, Nine Inch Nails and Peter Murphy of Bauhaus performed the Pere Ubu song "Final Solution", which was also a solo hit for Murphy. For the last show, they collaborated to cover Joy Division's "Dead Souls", which Nine Inch Nails has regularly played since 1994.  Also during this tour, Reznor, Murphy, and other musicians performed four unique sets of their favorite songs on radio stations around the country.
Ben Weinman and Greg Puciato of The Dillinger Escape Plan joined Nine Inch Nails on stage during the encore of their performance at the Adelaide leg of the 2009 Soundwave Festival to perform Wish and joined again on the Perth leg with the full band. On June 15, 2009 The Dillinger Escape Plan joined Nine Inch Nails on stage at Bonnaroo performing "Wish".
During the Wave Goodbye tour in 2009 Nine Inch Nails invited many special guests to perform with them.  The guest appearances included Gary Numan, Peter Murphy, Atticus Ross, Danny Lohner, Mike Garson, Dave Navarro, Eric Avery, Saul Williams, The Dillinger Escape Plan, and Health.
Wave Goodbye tour