The easiest smartphone stand might be the one that's already riding in your wallet. All you need is a business card -- preferably a thick one, as lightweight paper cards have trouble holding today's bigger, heavier phones -- and a few simple folds and cuts. (Scissors are optional but recommended.) There's a complete diagram over at Instructables.
Instead of throwing away the cardboard sleeve that stood between your hand and your piping-hot venti latte, do a little strategic tearing to turn that sleeve into a sturdy smartphone stand. There's a quick tutorial over at Instructables, but the photo really says it all: just rip off the top half of the front section, then add a couple of divots for the phone. This same principle would work with the coffee cup itself.
Two binder clips enter. One smartphone stand leaves. So pull a couple clips off of those business reports you've got stashed in your briefcase, then stick them together as shown. The only tricky part is bending up the end of one of the clip handles to use as a phone stop.
Stop! Don't throw away that used-up Starbucks card. Or hotel room-key. Or even expired credit card. In about 10 seconds you can turn just about any such piece of stiff plastic into a killer stand. One strategic fold here, a second one there, and boom, you're done. This is a great way to recycle a used gift-card, and one of my all-time favorite stand options.
If you have a scissors handy, there's another way to turn a gift card or other similarly rigid card into a smartphone stand. Just cut away one of the corners, then cut a notch into the corresponding edge. Make it just wide enough to accommodate your phone, and presto: a secure, custom-fit stand.