Spare amp and also Hartke VXL di/preamp thingy just in case taken to every gig.
Its being professional i think - we're getting paid to do a job and not let anyone down on the night so we cover most potential problem scenarios. Drummer has a spare snare, guitarist has 2 amps in a roadcase and 2 cabs, i have 2 separate amps and cabs in case one blows, we all have a spare guitar.
We did a big gig in Cornwall last bank holiday which involved a lot of travelling, big audience, good pay and got to thinking "what if an amp blows at 10pm - we're stuffed!!" so made a few investments since then. Good for peace of mind i think.