Some of the apps I use are:
BBM - to keep in contact with friends and family.
IM+ Lite - I use this as an IM program for Facebook, Twitter, Gmail, Yahoo, Aim, etc.
Skype - Depending on your model you may need to download iSkoot for Skype.
Facebook - Self explanatory.
SmrtGuard - additional protection for my phone.
Bolt - Another alternative to the built in browser.
Navita Translator - Language translator tool...not always accurate though.
BeBuzz - You can get a free vs on App World and it allows you to customize your phone.
Twitter - Self explanatory
BlueSlate Converter - Self explanatory
Lexulous - There is a lite vs and it's similar to Scrabble.
Pixelate - A cool game but gets a tad boring.
Just a word of advice...remember the apps may take up space on your phone's memory (depending on your model) which is limited.