Apart from the basics like Dialer or Messaging (although I prefer Handcent for texting) and Contacts, Music.....I would say:
My new most important and used app. AirDroid.
Top 5 - IMDB (Internet Movie DataBase), Network Discovery (tool for scanning networks to find computers and other devices), AppBrain (I use to update all my apps, and also has a feature where it searches all the apps you have, and builds a list of recommended apps based on the other apps you already have loaded), UnBlock Me (puzzle game), Camera360 (different Camera app that has a lot more features than the default app does)
Have been using Twitter a lot recently, but I figure it's not really a needed app, because I could just visit the Twitter website using a Browser.