Aside from all the usual necessities in no particular order...
1. Where's My Droid (found my phone a couple of times and from a Chili's employee who was hoping I was not coming back for it)
2. Llama (Awesome location awareness app that syncs my phone during different events like arriving home and phone is on AC power, even silents my phone automatically when we go to the movies.)
3. ES File Explorer (Allows me access to my home server to send music, photos, and other files back and forth)
4. Tinycam monitor (Hooks up to my ip phones around my house to monitor if I get an alarm, worked awesome while I was on vacation and could tell my neighbor to pick up my package that just got left at the door)
5. Evernote (Captures everything from pics to audio and feeds it to my Getting Things Done method of task management)