(Only FREE apps are listed. Because I'm a cheap bastard, I'm assuming you are too)

APP WORLD

App World is basically an App store for your BB. It's so much easier to get your apps through this client instead of trying to search for it on the internet. (Most of the apps listed here can be downloaded through App World)

Simply go to http://mobile.blackberry.com/ from your mobile browser and download App World.

Facebook

Upgrade to the latest version of Facebook for your BlackBerry (1.6).

Visit http://na.blackberry.com/eng/devices/features/social/facebook.jsp and enter your email address to download from your phone.

Flickr

Upload mobile photos to your Flickr account directly from your phone.

Visit http://na.blackberry.com/eng/devices/features/social/flickr.jsp and enter your email address to download from your phone.

Google Maps

Directions (by car, foot, or transit). Also has traffic and location sharing.

Visit http://www.google.com/mobile/default/maps.html and enter your phone number and Google will send you a text with directions for download.

Google Mobile

Search by voice! Also, search by My Location, which allows for search for nearby businesses.

On your mobile browser, go to http://m.google.com.

Google Sync

Sync your calendar and contacts. You can make updates on your phone or on the web (google account required). Visit http://www.google.com/mobile/default/sync.html and enter your phone number and Google will send you a text with directions for download.

Pandora

The internet radio app is now available on the Blackberry (except for The Storm, but they're currently working on it!). Make sure you have an unlimited data plan.

OTA available at pandora.com

TwitterBerry

Post updates to Twitter using this client instead of text messages.

OTA available at http://orangatame.com/ota/twitterberry/

TwitPic

This isn't really an app, but I'd recommend if you use Twitter. Using TwitPic, you can take a picture with your BB, email it to a special email, and it'll post to your Twitter. No need to signup, just use your existing Twitter login.

http://twitpic.com/

Vlingo

Talk to your phone! Features include: Send SMS, VoiceDial, Update Twitter, Update Facebook, Save Memo, open web browser, and more!

Visit http://www.vlingo.com/ and enter in your info to have a link sent to your phone