Geospatial

c:geo – Free, pretty good

Geocaching – Paid, but extremely well written. Use c:geo if you rarely cache, but if you’re a regular geocacher, buy this.

Sensors -

Tricorder – Free, uses virtually all of the sensors built into an Android phone

Utilities -

SuperBox – Multipurpose utility, I use it mainly for quickly checking my battery and moving apps to the SD card

MyBookDroid – You can use it for many purposes, but I use it to quickly scan/catalog my book collection

WordPress – Bit obvious, this.

ConnectBot – SSH client, tiny letters but handy for rebooting a server or restarting a service

ColorNote – Best notepad app I’ve found thus far

ElectroDroid – Multipurpose electronics utility, has all kinds of reference material

Diaster Alerts – Good way to check on world wide alerts of bad things

How To Tie a Tie – I’m not much of a tie person, so this is surprisingly handy

KnotsGuide – Very handy

Net Scan – Works alright, scans a wifi network

Net Swiss Tool – Various tools that are common on OS’s (ping, tracert, etc)

Wifi Analyzer – Has a handful of utilities for scanning wifi networks

SSHTunnel – If you are using public Wifi, you want to secure your traffic. This is the best way of doing so.

Where’s My Droid – Handy for “Where did I leave my phone” situations

Entertainment

Amazon Kindle – eBook reader. I just use it for free classics and reading books from Baen

Slacker – Internet radio

Khan Academy – Educational classes on just about anything