Sub DiggerPlus is a new tool that lets you more easily view sites your friends have submitted to Digg.com. To use it you just drop in your Digg username and it lists all of the stories your friends have submitted in chronological order over the past 24 hours. You can then sort by topic, submitter, and the number of Diggs each story has. The real fun of the service though, is a feature that lets you hop to each site with back and forth buttons, emulating the feeling of navigating an old Web ring.
Compared to Digg's own friends tool which links to each story's Digg page, the navigation bar on Sub DiggerPlus makes the process of visiting each site they've submitted quite a bit easier since you hop to the site and not the Digg page that links to it. You may be missing out on some of the Digg flavor, like user comments and critiques, but for new submissions that's not so important since you're likely to be one of the first people to see it.
What is important, and what this app does a great job at highlighting is the content itself. If you think a story is cool, you can hop back to Digg to vote it up. If not, you just click the Next button and you're on your way to something new. It's also smart enough to keep track of what you've read and what you haven't, as well as getting rid of any stories after they fall outside of the nebulous 24 hour period where they can make it to the front page after being submitted.
The tool is missing a few features that the Digg API will eventually be able to allow including the capability to Digg up or bury stories. For now you have to do that back on Digg itself, which can be a pain. It also gets rid of the source URL in your browser's address bar, which means you have to grab it from the navigation bar that floats on the bottom of the screen. This isn't terrible behavior, but for some users it may be a little confusing.
Otherwise, if you're a Digg power user, this is a fantastic tool for streamlining your story browsing workflow. When it eventually includes the capability to vote on stories without leaving the navigation, I think it will be a real keeper.