I was out buying some wood at Lowes the other day, when I realized that they needed a line on the wood to cut along. Naturally, I had forgotten my ruler or any other longish straight object. Just when I was sure that all was lost (and that I wouldn't be able to fit my wood in the back of my car), I realized that I had a library card in my pocket. I was able to use my library card and a couple of points on the wood to get the wood cut and in the back of my truck.
This method isn't suitable for really wide things, but my library card is good for anything up to about four inches in width.
Step 1: You Will Need...
A library card, credit card, or drivers license. Basically, it can be any piece of flat plastic that you might have in your wallet or purse.
Something that has two points on it
Step 2: Make Two Points
Just like the title says- make two points on whatever it is that you are measuring. If you already did this, skip this step.
Step 3: Library Card + Points = :)
Place the library card next to the points that you made, so that its edge touches each point.
Step 4: Ink
Run your pen along the edge of the card, then pull the card away.
Congratulations! You now have a straight line!
You could also place marks at different lengths along your card, to create a portable ruler.