No not the American Express card, though they are pretty cool. I'm talking about a speedlight. I don't care if it's one of the modern and amazingly cool/powerful Nikon SB800/900 or Canon EX580II or an old flash like an SB-24 or even a Vivitar 285. Even when I'm heading out with my case of monolights I always take two of my SB-800's "just in case". I rarely pull them from the bag but when I do they make neat-o things happen just because I can put them places that I could never stick a big strobe.
Grand example: I was shooting a story about Ana Weir who runs a non-profit that collects and reconditions used running shoes and then donates them to people in Africa. We were making photos in the laundrymat that she uses and I loved the light coming from the wall of windows on the far side and how it was reflecting off the stainless steel of the washers. I had her open one of them, crouch down and start putting shoes into it. However with her being dark skinned and facing into the washer her face was way too dark. So I put a medium softbox at camera right and a strobe with a grid over her shoulder to add definition to the scene but it was putting an SB-800 into the washer that made it happen. There was no way to get a monolight into the washer even with my Innovatronix battery pack so the SB was the only option but it was one that I could take advantage of.
I wanted a limited a limited depth of field so with my AF-D 85mm f/1.4 set to f 2.8 I didn't need much power from the SB at all. I put the dome diffuser on it and attached a Pocket Wizard using the shiny stainless interior as a big reflector. The rest was easy but getting that little strobe in there made it all happen.