brainstrom mode on:
* Random shield
Provides real random data (through I2C or SPI), something like an radio that receives and converts noise to bits.
* Voip shield
A shield that can connect to VOIP phone calls (not the audio part but to interact with connection stuff)
* Phone charger shield
A shield that controls the charging of a mobile phone (monitors the load)
* landLine telephone Switch shield
Can switch on/off landlines, optionally with a RTC
* Serial Shield
Provides multiple (4) HW serial ports with big buffers
switchable to RS485?
* Power supply shields
Solar ? will it provide enough?
Backup battery (UPS alike)
Battery Shield (Holds 2x3 AA) - rechargeables prepared
* Cover Shield
A small box that can fix an Arduino by using the shield pins (top part can act like an umbrella

* Status shield
Has for every line a LED that shows if it is LOW/HIGH (dimmer if PWM or an analog voltage)
e.g. shows (implicitely) SPI I2C communication
* Light Dimmer shield
Control 4 lights in a living room, => in general a Domotic Shield
* Navigation shield
Holds compass and GPS
* Meteo shield
with room for - humidity, temperature, airpressure, airquality,... sensors
enough for coming months

I second Lefty's DAC / ADC is a good one,
also second the multicore-shield from cr0sh