It really depends on how strong your school is, or how strong it is perceived to be. At Stuyvesant in New York, or Thomas Jefferson in Virginia, or Scarsdale High, or Groton, top 5% is good class rank, and top 10% may be good class rank. At East Podunk Vo-Tech, the valedictorian probably doesn't have much of a shot.
By any real-world standard, being ranked 15 out of 500 is great, and you should be proud of yourself. To get a rough sense of what it means for college admissions, though, you probably have to look at what has happened at your school in the past. Where do people ranked 10-20 get accepted? That doesn't completely predict what will happen with you, of course, but it will give you some idea of how things tend to pan out.
My kids' public magnet had classes of about 550. It regularly sends one or two students to Harvard, Yale, sometimes Stanford (hardly ever Princeton) . . . but in recent years at least no one ranked lower than 8 or 9. Kids in the top 20 or beyond would often get into Penn or Cornell, though. But each school is different. There's a smaller public magnet here that sends slightly more kids to HYP; there the absolute rank range is a little bigger, and the percentage rank range obviously much, much bigger.
[Cross-posted with Admissions Addict, saying the same thing as he with less concision.]