I've been playing with this build for a little while on my Windows XP partition, and I already miss it when I'm forced to use an alternative machine. The combination of tabbed browsing and thumbnailing, RSS feed subscription, filtering and searching, integrated search, page zoom and trustworthy printing all make my life easier as an end-user, and the enhanced accuracy of CSS and XHTML rendering will be a great relief to developers everywhere. (And did I hear "transparent PNG"?)
Download it yourself from the Microsoft Windows Internet Explorer 7 home page. Note that this is a preview of the beta and is intended for developers and technology enthusiasts, rather than people like my mother (who wouldn't ever describe herself in either of the above categories!)
Oh, and here's the obligatory WPF-relevant piece - try clicking on a .xaml or .xbap link with IE 7 installed... see what you can discover.