In America, specifically in the Midwest where I live, if I want to leave a job, I walk out. I've seen people go to lunch and not come back in extreme cases. I almost always have given at least a 2 week notice, but there is no law requiring me to give any length of notice. It's seen as a nice thing to do, and in some industries, like IT, where we tend to know a large network of people around the city, it's easy to get a bad reputation if you don't leave on good terms.
I keep reading questions from other people where several weeks are "required". Is it actually a law in other places in the world? Do companies not have a right to hire or fire someone in some places? In my state, it's known as an "at-will" state. My company can fire me for any reason. They are not forced to keep me on if I'm a bad employee. Nor am I forced to remain working for a company I don't like.
I'm curious to know if there are any countries represented by posters here that have laws that actually require it. In the USA it just isn't a requirement that I've seen. Certainly not in the few states I've lived in.