Amazon Appstore app.
Amazon.com: Appstore for Android. Another great place to look for apps. They have paid and free apps, and, every day they offer a feature paid app for free. They are generally games, but, still pretty awesome. Every couple of weeks or so though you get a relatively useful app though up for free. I make sure to get the free app every day. And, it stays with your account, so, once you "purchase" it for $0.00 you don't even have to install it. I just purchase every day, and, don't bother installing. Then if I ever need it, I can install it for free!!
Google Voice for free text messaging.
Astrid Tasks.
I like The Weather Channel app better than the one that came with my X2. The widget is nicer, and, the app seems more functional.
Cozi. Lets me and my girlfriend share a shopping list (among other things, but, we only use it for shopping). There are other apps out there that do this, like, Out of Milk. But, we got Cozi for free as app of the day from Amazon Appstore. Out of Milk you have to pay for the version that you can share lists. Either way, if you have multiple people shopping in a household I've found this tool to be invaluable. They have a website as well where you can manage your lists. So, I manage my lists all week long in a browser from a PC, and, then use them on my phone with the app at the store.
Bluetooth File Transfer is handy for transferring files and apps to someone else with a smartphone.
UPnPlay is nice for streaming video from your computer to your phone. You need to use a media player in conjunction with it though. I use MoboPlayer, QQPlayer and Rock Player.