Not everybody has a camera with interchangeable lenses, but if you do than you should have a 50mm f/1.8. We'll start with the simplest reason, it's cheap. You can usually find it for around $100 dollars used. With it you get a lens with a great shallow depth of field (that fancy blurry effect in great photos) like this:
Also, it gives you a fantastic lens for portraits like this:
Lastly, low-light photos like this:
This is not to say that this is the only lens that you should use, but certainly one of the most versatile lenses that you should own.