As I mentioned a few days ago I've been working on changing how posts are tagged here. Now, for each post, you can see how everyone else has tagged it in del.icio.us. This is much cooler than seeing just my tags. I find it quite astounding how many of my posts have been bookmarked and even more astounding the huge variety of tags used. They're also informative. For example, somebody thinks this belongs in the bad-ideas bucket. Cool.
Microcomments. Made possible by Web 2.0.
For those more technically minded, I did this via a custom wordpress plugin which fetches the RSS feed for a given URL and parses it looking for the tags described by the RSS taxonomy extension. I collect all these together, count how many of each I have and spit out the list. It caches lookups so as not to hit del.icio.us too hard.