Nightmare Cinema is a changed up version of Dream Theater where all the members changed instruments and jammed on songs by Deep Purple or Frank Zappa. Oftentimes the band would use this to open for themselves or as an encore.
The lineup to Nightmare Cinema is as follows:
Recently at one show DT switched instruments again for an updated Nightmare Cinema in which they played a Zappa tribute. I'm no sure of the exact lineup although I do know that LaBrie played drums.