My friend Xin Lu sent me a bunch of blister packed CR2032s. Note the price is for five at a time. These are great for small Arduino projects, and they scale nicely if you need more voltage. I have used 2 and called it 5 volts and also run 3 of them through a regulator to get my necessary 5 volts. You can use the actual clip holder for these, or my personal favorite is to make a roll out of them and electrical tape it, leaving the bottom and top ends open to attach the leads. These are ideal for small applications where space is at a premium. Sometimes I sit around and wonder how many of them it would take to jump-start a car!