If you are taken as a given that we have spare leads, batteries, strings etc. I would say that I take spare fuses. Of the larger items; I have spare amps and cabs at home (to get me through the next gigs till things are fixed) but I take a Sansamp pedal which I have in-line and I've been running an XLR line-out to our PA (vocals, guitar [guitarist has no backline] and kick drum), which I can switch in 'IF' my rig went down. It only takes 2 minutes and 1 XLR cable to set up my back up.