Adventurous users - remember: this isn't even a beta - can check out the change notes for this public sneak peek, and head over to the download page to give it a go. For a tasty primer, a lot is new: Spotlight searching, a much-requested new Combined View, a Full Screen mode, a tab bar on the right with page thumbnails (which can be condensed to mere text titles), the ability to chose a folder when subscribing (a lá Safari bookmarking) and much, much more. As a personal side note, it's great to see even that FeedLight style Brent found recently to make its way onto the built-in styles list.
A few words of caution from a blogger who's been testing this version for around a month (hint - that's me): back up your existing NNW support folder if testing software like this makes you queasy. NetNewsWire 3 makes significant changes to the way your data is stored, and you won't be able to run version 2.x without its old support folder after firing up v3. Surfing to ~/Library/Application Support, selecting the NetNewsWire folder and pressing cmd + d is a simple enough backup.
With that said, go enjoy a sneak peek of NetNewsWire 3.