You Can Now Download OS X Betas Legally Without A Developer Account

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OS X Beta Seed Program
Hitherto, those looking to test out beta releases of Apple’s OS X would need to sign up to the Developer Program, which, for $99 per year, looked quite a steep fee for the casual, inquisitive user.
Now, however, the Cupertino company has introduced a new OS X Beta Seed Program, whereby users can test out pre-release builds of the Mac software simply by signing up with an Apple ID – no annual fee required.

1
Launch Feedback Assistant
Once you have enrolled your Mac for beta updates and installed the OS X 10.9.3 beta, you will find Feedback Assistant in your Dock.
Enroll Your Mac
2
Sign in with your Apple ID
Sign in with the same Apple ID that you used to register for the OS X Beta Seed Program.
3
Give us your feedback
Reporting any issues you encounter is easy. Just fill out the step-by-step form, review your answers and diagnostic files, and send them to Apple.

https://appleseed.apple.com/sp/betaprogram
https://appleseed.apple.com/sp/betaprogram/landing

Usual caveats......
Note the emphasis on the word..... Beta.
OS X Beta Seed Program
Backup, and backup again before downloading, and during Beta Testing

And to return to your previous version of Mavericks
OS X: About OS X Recovery
OS X Lion, Mountain Lion, and Mavericks include OS X Recovery.
This feature includes all of the tools you need to reinstall OS X, repair your disk, and even restore from a Time Machine backup without the need for optical discs.
http://support.apple.com/kb/HT4718