The other day I was searching for some advice on Ruby on Rails. I got the usual links to message forums, experts exchange, HOWTO articles, documentation and API pages. One of the search results I found was a link to a twitter account. It seemed out of place among the other resources, but I soon discovered that this user is essentially using Twitter (via hash tags) as a gigantic, distributed message forum. And he's not alone.
Just throw a bunch of hash tags (#rails #ror #ruby #gem #railsgem) on your tweet and it immediately shows up in the "message forum" (hash tag feed) for the given topics. A "post" (tweet) can be cross-posted to multiple "forums" (hash tags) at the same time. Replies come directly to you via your @username. The main disadvantage is the 140-character limit and the inability to include formatting or other media.
I've never gotten into Twitter much, but this realization presents a novel use of what I had come to think of as simply a marketing platform. Maybe it's got a broader use after all.