The now ubiquitous hashtag, which greatly extends the fun of Twitter by allowing the spontaneous grouping of tweets, did not emerge from inside the company. It emerged from the community, and it has a born on date, as Steven Johnson highlighted last night.
That date is August 25, 2007, when Chris Messina (now of Google) wrote this post, laying out the basic idea of the hashtag. It makes for a fascinating read now, largely because Twitter has changed so much. Back then, it had a few hundred thousand users, most of them the geekiest our society had to offer. Here's his elevator pitch:
What I've realized is that this "channel" concept meets many of the aggregate desires expressed in various "Groups for Twitter" discussions while not inheriting a lot of the unnecessary management cruft that most group systems seem to suffer from, it is easily accessible adapting current Twitter syntax and convention, it's easy to learn and lightweight, it's very flexible and entirely folksonomic and works with people's current behaviors, rather than forcing anyone to learn anything radically new. It also keeps the interface aspects to a minimum (as I'll soon explain), invents little by borrowing from age old IRC conventions also adopted by an existing web application and, from what Britt said so far, actually works consistently on cell phones (whereas, for example, the star key does not).
The hashtag didn't catch on right away, according to Liz Gannes' history of the hashtag, but now about 10 percent of all tweets feature the #tag.