johngineer was tinkering with Arduino and came up with fun Arduino implementation. He managed to draw a Christmas tree right on Oscilloscope screen.
Original design is built around Attiny2313 microcontroller running at 16MHz. To do a drawings on scope there X and Y oscilloscope axis has to be controlled. For this two PWM channels are used that are filtered with simple RC low pass filters. This way managing analog voltages it is possible to do simple drawings. There is also an example implementation using Arduino but in my opinion using Tiny2313 is more cost effective solution. BTW, best results are seen on scopes with CRT screens due to screen properties, but should be OK with LCD based ones.