I'm just coming across the idea of hashtags for creating groupings on Twitter after seeing them in factoryjoe's tweets and following a link from merylkevans to the page Twitter Fan Wiki on hashtags. This is a great example of how new uses of an application emerge can be defined by users.

Hashtags are a community-driven convention for adding additional context and metadata to your tweets. They're like tags on Flickr, only added inline to your post. You create a hashtag simply by prefixing a word with a hash symbol: #hashtag.

Hashtags were developed as a means to create "groupings" on Twitter, without having to change the basic service. The hash symbol is a convention borrowed primarily from IRC channels, and later from Jaiku's channels.