Blackberry Alerts is hands down the most useful program I have ever added to my device.
For some reason, RIM thinks it's enough to just notify one time on events. So, if you get an SMS and don't hear or see it when it comes in, you have to wake your screen to see that you got it. There is no indication that you should attend to your phone. Every other phone I have used keeps bugging you until you acknowledge it.
Blackberry Alerts fills the gap. At $10, more than worth it. I had it on my Pearl and now my Curve. As far as I know, you can transfer the license too (see below).
JiveTalk... Probably the best IM client.
I am a little irked by their two tier pricing structure. I think when you buy a piece of software, you should be able to use it on any single device forever - similar to how you buy software on a PC. By paying $20, you are gambling that your device won't break. It isn't that much money but it's annoying.
Imagine if you bought Quicken or WOW for your PC, upgraded to a newer PC as many of us do, and had to buy the software again... unless you paid an extra "insurance" payment.
Oops... sorry... lost myself. Both programs are excellent.