Apple has introduced its OS X Beta Seed Program and made it available for everyone. Anyway willing to test the latest pre-release builds before they are officially launched.
Generally, users needed a $99 developer accounts to test pre-release builds from Apple. Now in order to test the latest build, all you need is an Apple ID which is completely free. After signing up, you can test the latest Mavericks pre-release build and submit feedback to Apple. Even if you don't feel like giving any feedback and just wish to use them, that is also fine with Apple.