A long time ago I made an HP spectrum analyzer display a TV picture. Couldn't hear the sound but it was a fairly decent black and white picture (well green and white). We also used to make oscilloscopes display patterns. I thought I'd try it with the Arduino.
Here's three I got going using the Arduino - square, diamond, and circle I only used passive components and programming.
I got that scope back in 1986, and I paid $461 for it. A Rigol DS1052 costs about $330. However, this scope is pure analog - no "funny business" going on inside that baby! But ... I'll probably buckle and get a new digital scope, I mean the screen colors are so pretty.