How to Download and Install the Minecraft 1.8 Pre-Release
Notch posted the pre-release of Minecraft 1.8 before it got an official release. If you want to install Minecraft 1.8, take a look at this tutorial for instructions on adding the update to your Minecraft folder in Windows on your PC. You can download the pre-release on the Minecraft forums in this post.
The 1.8 pre-release is a little buggy, and will only work if you've actually paid for Minecraft but otherwise, this a great way to play the 1.8 Adventure Update early.