Blackberry, with the launch of their own Blackberry App World in April, has been gaining some ground on the iPhone in "isn't-this-cool?!" functionality...
Probably the most universally useful application we've seen from Blackberry is the Call You Back app.
Absolutely free, this app will send out a pre-selected SMS text message to your ignored calls. If you're in a meeting or a movie, you can set Call You Back to send a custom text letting all your callers know.
But we love this app for it's usefulness when you're not just ignoring calls, but missing them completely. If you're visiting your cousins in rural Montana where your cell service is slim—or you're just in a dense building and don't want your boss to think you're ignoring her—your callers will know "Sorry, I'm in a location where I'm getting spotty service. Call me back in a few or leave a message!"