Apple has rolled out the OS X Yosemite public beta. The program was announced last month at WWDC and allows ordinary users to test a prerelease version of the operating system before its general release in the fall.
Users can obtain a redemption code to download the beta from the Mac App Store at this link: https://appleseed.apple.com/sp/betaprogram/. Once there, if you've already signed up, click sign in and enter your Apple ID. If you haven't, click sign up to become a member.
Once logged in, you'll be required to accept an agreement and then be issued a redemption code. Click the link and you should be directed to the Mac App Store.
It seems like early adopters are getting an error with the code already being redeemed. If this happens, try refreshing the page to get a new one.
OS X Yosemite Public Beta Protected by NDA
Please be aware that the Mac OS X Yosemite public beta is protected by a Non-Disclosure Agreement (NDA) and is considered Apple Confidential Information.
In particular, Apple states the following on its OS X Yosemite Public Beta Frequently Asked Questions page (https://appleseed.apple.com/sp/betaprogram/faq): "Is the pre-release software I am installing confidential? Yes, the pre-release software is Apple confidential information. For example, don’t install the pre-release Apple software on any systems you don't directly control or that you share with others, don’t blog, post screen shots, tweet or publicly post information about the pre-release Apple software, and don' discuss the pre-release Apple software with or demonstrate it to others who are not in the OS X Beta Program. For clarity, if Apple has publicly disclosed technical information about the pre-release software then it is no longer considered confidential."
For more information on testing OS X betas as a VoiceOver user, check out this guide by Mehgcap.