This one started when I found one of those simple lissajous kits I bought years ago. It has 2 DC motors with mirrors on them, you can control their speed and direction with 2 potentiometers. This is unusable if you really want to put the laser as a light effect somewhere, so I build a small controller unit for the motors and the laser. I also replaced the red laser with a green one since they got quite cheap now, are brighter (for the human eye) and the ray is visible even with just 1mW due to the greater rayleigh scattering.
The controller is based on a ATTiny26. Two PWM channels are used to set the motor speed. The direction of one motor can be reversed, this is done through a dual relais (I lacked enough PNPs to build a h-bridge). The laser is a green laser pointer, a LM317 is used for the 3.1V power supply, as well as a small FET to switch it on and off.
Nothing spectacular, I just use a dual opamp to amplify a microphone with variable sensitivity. The output goes through a simple (3dB 150Hz) low pass to the ATTiny26 ADC input, where a free running ADC sets the speed of one motor, while the other motor runs at a fixed speed.