30 Boxes has, or will have, the blogorati in a tizzy shortly. The web app is a calendaring tool that apparently will employ many of your favorite Meme Who's Name Shall Not Be Uttered features, slickly executed: tags, AJAX, social mechanisms. Pseudonymous tech chronicler Thomas Hawk has one of a canonical overview of 30 Boxes in its current pre-release state.
One interesting point of 30 Boxes is the mechanisms it uses to implement social boundaries. From my read of the review, access control is implemented using a combination of buddy lists and tags. I've been thinking that buddy lists are the only widely adopted and accessible tool for end user access control and should be applied in just about every piece of social software. The combination with tags though is just brilliant. The tags make it relatively easy to say what elements of your personal calendar others can access. In fact, the tags support a flexible grouping mechanism that should allow users to settle on the right groupings for their usage. No tyranny of one size fits all granularity as in UNIX's owner/group/other model. We'll see how the combination holds up when it meets the real world.
Also, as folks start to realize that tags make a great collecting and labeling tool, we'll see more applications besides the holy manna of bottom up categorization attributed to folksonomies.
You know I've always liked that word... "pseudonymous" ..., so rarely have an opportunity to use it in a sentence