Blackberry messenger, which is simply texting from Blackberry to Blackberry...
I've had mine for a couple months, and I find something new nearly every day...
Play with it and see what you can get it to do... Download Blackberry App. World. There are several cool free apps, and the majority of the rest are pretty cheap.
I came to the Curve from the Treo755... There are things this will do the other phone wouldn't and there are things the other would do that this won't... Thus far it's been a pretty good trade off.
I also would look at getting "Bolt" it's a browser for the phone, it is far better than the BB, browser. Keep an eye out for Skyfire, it's supposed to even be better than Bolt. It's still in Allpha testing, so it should be available for Beta soon. Then there's always Opera Mini, which I never liked, but it still works better than the BB Browser.