I love my BlackBerry 7250, my personal communication device from Verizon Wireless is invaluable. I'm in the process of switching over to a company provided Blackberry from Cingular.
Right now I can't install any third party applications, but hopefully I can figure this out soon.
What's On My BlackBerry?
Google Talk - If you have a Gmail account, you're already aware of Google Talk. The nice feature is that it does not use SMS to send and receive messages. It uses the data connection and with an unlimited data plan you're covered. It even works with Twitter.
Google Maps - This installs on your BlackBerry and gives you access to normal maps, satellite, as well as traffic. You can search for business and get directions. It's super helpful and just like Google Talk, it's free and piggybacks on your data plan.
Google Gmail - This is an installed application which gives you quite a bit of functionality to read and action upon your Gmail.
Google Reader - In the last couple months I've embraced RSS and have used feeds to help my productivity and keep me in the loop on various things. Google's Reader is easy to navigate and I can catch up on my feeds while commuting on the train or riding in a taxi. I need to check out the AOL Feedreader, from what I've seen it has more customization features.
Amtrak - Essential if you travel the Northeast corridor.
Southwest - Essential to ensure you get your A boarding pass. Check-in through your Blackberry.
FlyFAA Plain Text - Great way to find out if there are weather delays on your flights.
HopStop - Essential for navigating the subways in NYC and other major cities.
MapQuest - If I need directions displayed all at once, MQ is great. MapQuest is my standard computer mapping choice, but for quick looks at a map I'm using Google Maps on my BlackBerry.
AOL Mail - When I need to check my non-work AOL account, I would equate the interface to be comparable with Google's Gmail web interface.
I'm curious to know what other people find essential on their BlackBerrys.