Read all about and download it  here:

http://www.microsoft.com/windows/internet-explorer/beta/

I have been running Beta 2 this morning, and it seems noticeably faster.

If you really want a good insight into IE8, check out the IEBlog. This link will take you right to Paul Cutsinger's  post where he introduces IE8 Beta 2. Paul is Lead Program Manager for the IE8 User Experience, and given how important the browser has become, I think this is well worth reading.

Link: IEBlog and Part II of that series

Installation Notes

I had been running IE8 Beta 1 and it was working fine for the most part. Today when I downloaded Beta 2 and tried to install it, it required that I uninstall Beta 1. Let me save you some time if you are in the same situation.

Go to

Start

Control Panel
Programs
- Uninstall a program

NOTE:
click -> View installed updates
You will find it under

Windows Internet Explorer 8 Beta  (if you are running Vista - search for "inter". It will come up right away).