Now, what I find interesting about this application is that although it's one of a zillion Twitter-related apps, it's actually useful. It's useful because it addresses a very specific need: the need to comment and discuss the movements of the stock market as they happen. You'll see it on every stock-market related forum: the busiest topic is usually one that follows current trading.
StockTwits replaces exactly this forum topic; it might seem like a very tiny niche, but it's not. There's a very real, actual need for an application that replaces this functionality with something better - something faster, easier to browse through and filter, something that has nice graphs and other features that forums traditionally don't have. The folks that built StockTwits have recognized this and they've built - probably on the cheap, since they use Twitter's infrastructure - a nifty little application that's bound to find its audience.
What's next for StockTwits? Well, the application is useless for me: I don't trade in US stocks. Making it work for other stock markets as well would be a top priority. But still, if there's one Twitter-related app I'm betting on, it's this one. Time will tell if I'm wrong.