A small fan, driven by a hobby motor. Powered by 2x AA rechargeable batteries. The batteries are hooked up to a solar panel, which recharges them during the day.
The circuit is switched by a CdS cell (light switch) which means the fan switches on when it is dark enough, at night, hopefully.
Only after I'd built the (incredibly ugly - sorry, no photos available) prototype did I realise I could've bought a pesonal fan for $5 and hacked it instead.
Here's the schematic. Feel free to use it or improve upon it to your own satisfaction.

Step 1: Parts, Clarification, That Is.

The transistor is an ordinary NPN 100mA 45V.
The CdS I hacked from a solar garden light.
The solar panel came from the same place.
It's output is (rough estimate) 4.5V, 60mA.
I used an old hobby motor with a range of 2-6 volts. No special reason, it was handy. And the propellor came from a model plane.
That leaves the diode, 1N4004, and the two resistors, 1k and 10k.

Step 2: Other Uses

This isn't really a step, more a note about other things I've done with this - putting a white LED driver circuit in place of the motor.
A variation of the idea of the solar garden light I got the parts from originally.