Niches

arduino based automated plant growing
-soil moisture sensor for automated irrigation (relay output)
-temperature sensor for thermostat
-relays for timing night/day cycle
-humidity sensor for keeping correct humidity (misting the plants/air)
-light sensor for keeping track of ambient light intensity (where do plants need more light)
-webduino for displaying data (possibly) - would prefer to use mysql backend with some form of php graphing library
-wireless sensor modules that relay to main "decision making" controller
-lcd for displaying real-time data
-I have all the circuits/components or can get them from work
-android app or web interface to manually control

arduino based alarm system
-same as above but uses:
-reed switch door sensors
-glass break sensors
-rfid cards to know who is home, when to sound the alarm
-PIR motion sensor
-can be expanded to be home automation project
-can use rfid and servo to unlock door automatically
-relays to turn on and off lights or other household appliances
-android app or web interface to manually control

arduino voltage meter
-voltage divider to measure voltage
-amp sensor to measure current
-RC time circuit to measure capacitance/ESR
-Resistance measurement