Recently I bought a GoGoStand for my iPhone 3G (I know, it's an old phone, but I don't have the money to upgrade very often). It's a little stand that folds up into the size and shape of a credit card, meaning you can easily keep it in your wallet.
This thing has been so useful for me. When I take lunch breaks at work, I can set this thing down on a table and watch videos while I eat. When I'm on a plane, I can set it on the tray in front of me.
It has 3 different viewing angles you can choose from, which is even nicer because you can use it on all kinds of viewing surfaces at different heights respective to where you're sitting. The website says you can put your phone (which, by the way, doesn't have to be an iPhone) in landscape or portrait orientation, but I've found that portrait is hard to balance and sometimes tips the whole thing over. The bottom of the stand isn't weighted, so it's best just to use landscape mode whenever possible, which you're likely to do anyway if you're using this thing to watch videos like I am.
I recommend picking one up, they're only like $5 and are perfect in a pinch.