To improve your chances of getting into a better school or to take a needed break from being a student. A year off can make a great college student out of a burnt out high school senior. Check out a program like Americorps or City Year . Get a job in a field that interests you. Even travel if youc an afford to before committing to four more years of school.
Many private schools offer a post-graduate year.
If you are financially unable to go to a PG program, you can take courses nearby at a community college part-time and have a job to pay for them at the same time. Apply to school now. Almost any school will allow you to defer your acceptance for one year. Some admissions officers even say that they like to see students take that year off - it indicates maturity and a strong interest in something. Just be careful if you want to take classes during that year - some schools have a limit as to how many you can take before you have to reapply as a transfer to enter the next year.