I'm surprised to see so many negative comments. It's not about neglecting your duties or pawning your work onto someone else. It's about doing your job quickly and efficiently. My personal opinion is many jobs should not be structured as hourly full time positions but should instead be based off your actual value and efficiently. So if you can get the work done in 10 hours a week that should be it. You get paid 4x as much per hour. Many companies are starting to move towards independent contractors who are paid on a more realistic and honest scale and compete against each other on value & efficiency. I think it's a good thing. Of course it does not apply to all office jobs and probably not even the majority of jobs. It tends to be more common with engineers.
Ah yes, I've had that dream too. Unfortunately, the 8-hour, structured workday is in place because it works well for average people. It would be hard to find a workplace where you can say to your boss "I got my work done for the day by 11:30. I'm gonna take the rest of the day off."
Actually, I did know one guy who worked at a place like this in Australia. He completed his project for the week by Tuesday and when he asked his boss for more work, his boss just told him to take the rest of the week off.
Unfortunately, almost every other place requires you to still sit in your seat for the remaining time.